|
|
By Paul Thurst, on April 26th, 2011
Eventually, disaster will strike. It can range from a fire at the transmitter site to a tornado at the studio. Someday, every station on the air will be knocked off at the worst possible moment. It is the law of nature. Perhaps the most difficult disaster to recover from is the loss of a tower at a transmitter site. An FM tower holds the antenna, therefore, finding a tower or building nearby and placing a temporary antenna there will get the station back on the air in a reasonable fashion.
An AM tower is the antenna, which is much harder to replicate. One possible solution is to use a temporary wire antenna while the tower is being rebuilt. This is allowed in FCC 73.1680 emergency antennas, provided the commission is notified of the situation by informal letter. Directional stations must operate at 25% or less of the station’s licensed power, or demonstrate that radiation limits are are not being exceeded in any direction. That usually can be accomplished by taking a set of monitor points.
A wire antenna can come in several configurations:
- Fastest to deploy is the random length end fed wire. This can normally be attached to the existing ATU and tuned up with components on hand. It requires having an OIB, generator and receiver to tune, which not every station has. In addition to that, extra components may be needed in the ATU for tuning purposes.
- Next easiest is a tower length wire ready to deploy. This is a length of wire equal to the height of the tower, with insulators and supports. Wire should be supported as high above ground as possible using trees, wooden poles, etc. Still requires having an OIB, generator and receiver to tune. Likely to be within the tuning limits of the ATU components on hand.
- A 1/2 wave dipole tuned for 50 ohms. This can be connected directly to the transmitter output, thus is the best solution if the ATU’s were damaged or otherwise not serviceable. In this situation, two 1/4 wavelengths of wire are coupled at the center using a 1:1 balun. Again, this antenna should be supported as high above ground as possible using trees, poles and other non-conductive supports. Can be installed in a V, inverted V, or L shape as required.
All three of these choices would likely limit transmitter power output to 1-2 KW. Choice 3 likely represents the most efficient radiator and can be fabricated ahead of time and stored at the transmitter site.
 1/2 wave dipole with 1:1 balun To make a 1/2 wave dipole, cut two lengths of wire using the formula L(feet)=246/F(MHz). This formula does not account for a velocity factor of 90%, which is typical for stranded wire. The reason being, since a MF dipole antenna is necessarily going to be lower than 1/2 wave length, it is better to start the antenna a little long and trim it to size for a 50 ohm impedance. If commercially made insulators are not available, insulators can be made from non-conductive materials like PVC conduit, PEX, plexi-glass, etc. The insulators on the ends of the wire need to account for the voltage peak that will occur there. If small “dogbone” type porcelain insulators are used, string thee or four of them together using nylon or poly rope. The insulator needs to be able to withstand 8-10 KW of power under full modulation.
A 1:1 balun will distribute the RF currents evenly on both wires, which will help improve efficiency and coverage. Most Ham Radio Baluns are not designed to work below 1.8 MHz and therefore, will not work for this purpose. A balun can be made with a ferrite torroid made from 68, 73, 77 or type F material. A good choice would be Amidon FT-290-77 or FT-290-F. The type F material has a higher AL value, thus fewer turns are needed. In addition to that, high voltage insulated wire should be used to wind the balun.
 1/2 wave dipole antenna current voltage distribution Since, in a 1/2 wave dipole configuration, the voltage is at a minimum at the center of the antenna, and current is at a maximum, some attention needs to be paid to wire size as well.
 Amidon ferrite torroid core To give a good idea of wire sizes required, some basic information is needed. For a 1 KW station, it is assumed that the carrier will be modulated to 100 percent, therefore the peak envelope power will be 4 KW. If the station is asymmetrically modulated, add another kilowatt. Therefore, the maximum current formula is I=√(P/R). P is the power in watts, or 5,000 and R is the radiation resistance or 50 ohms, thus I=√(5000/50) or 10 amps. The maximum voltage is E=√(P x R) or E=√ (5000 x 50) or 500 volts. For a safety factor, multiplying these values by 1.5 is recommended. That will likely account for any impedance differences due to ground proximity and so forth. Therefore for a 1 KW station, the dipole antenna should be designed for 15 amps and 750 volts.
For a 2 KW station the peak envelope power for an asymmetrically modulated transmitter is 10 KW, thus it follows that 30 amps and 1500 volts are safe working figures.
With the proper torroid core, a turns count of 7-10 turns bifilar will suffice. Since it is a 1:1 balun, the turns count on both sides of the transformer will be the same. The balun then should be placed in a suitable water proof housing designed to be attached to the center of the dipole antenna. This is a good example of a commercially available 5 KW 1:1 balun for amateur radio use:
 1:1 balun designed for center of 1/2 wave dipole antenna The antenna can be fed with RG-8, RG-8X, RG-8A, RG-214 or any other coax this is capable of handling the peak envelope power of the radio station. The connector can be UHF, N, LC, etc. In some cases, the may be easier to simply omit the connector and connect the coax to the balun using some type of strain relief on the cable coming out of the box.
Once this antenna is made, a bit of tuning may be required to bring it to 50 ohms. This can be done with a bridge and generator, or with the transmitter on low power. Either way, the measurements must be taken with the antenna at operating height as the distance to ground will effect the termination point impedance. It may require some trial and error.
In all, a good backup antenna can be made for about $50-60 or so. A little bit more if fancy transmission line is used. Well worth the expense and effort to have something ready to go in a moment’s notice.
Update: I’ve been fooling around with this on EZNEC, it may not be that easy to do, especially with the lower frequencies in the AM band. The antenna needs to be at least 0.06 wave length above ground to perform correctly. Somewhat lower over better ground conductivity, e.g. ground radials. Even at this height, it needs to be lengthened significantly to get the feed point impedance close to 50 ohms.
By Paul Thurst, on April 7th, 2011
There is a propensity among radio engineers to save old equipment. Sometimes I look at something and think, “Man, that cost a lot of money ten or twenty years ago.” Truth be told, much of what is saved will never be used again. This equipment should be scraped or donated to someone who might find it useful. One thing that is most appreciated by Amateur Radio (AKA Ham) operators are old 1 KW tube type AM transmitters. Ham operators love these things, and with good reason.
A fair amount of repair work, some cleaning and a bit of reworking will turn what might have been a useless dust collector into a 160 or 80 meter AM rig and with a good story to boot.
Personally, I’d rather see a Gates BC1T or RCA BTA1R off to a new home than off to the scrap yard. To that end, today we unloaded the BC1T at WLNA to a willing ham. This particular transmitter had last run in 2001 or so and was used as a spare parts supply for other BC1T transmitters owned by the same company. There was no way it would ever work again and truth be told, it really wasn’t needed any longer anyway. Since the Harris MW5B was replaced as the main transmitter by a BE AM6A, the backup transmitter was never used.
 Gates BC1T transmitter John Aegerter, a frequent commenter on this blog, drove all the way from Madison, Wisconsin to pick it up. Prior to pick up, I removed all of the tubes, transformers, crystals and glass envelope time delay relays. I packed up the glass objects in a box.
 Gates BC1T tubes, transformers and spares There were several spare tubes and parts which are no longer needed. These went with the rig, along with what ever manuals I could find.
 Gates BC1T loaded into pickup truck The transmitter was then loaded into the back of a Dodge Ram 2500 pickup truck and tarped for it’s trip back to Wisconsin.
By Paul Thurst, on March 21st, 2011
I received this link in the comments of a previous post and found it interesting. The BBC will be closing down 648 KHz, Ordfordness England at the end of March, no doubt due to budget cuts. The site has been in use since 1972. Prior to this, site was formerly an OTH array, COBRA MIST, which was then adopted for MW broadcasting. The video is 17 minutes long, but, if you are interested in radio history, technical aspects of AM broadcasting and the like, it is interesting.
Tribute to BBC 648 kHz Orfordness – The Enthusiast’s Version from Jonathan Marks on Vimeo.
These are 600 KW transmitters. As Andy Matheson, transmitter engineer, explains, with a wry smile “I find them (transmitters) very satisfying, I enjoy either day work or shift work, just really working with transmitters has always been very satisfying…” I couldn’t have said it better myself.
By Paul Thurst, on March 10th, 2011
Amplitude Modulation (AKA AM) was the first modulation type to impress audio on an RF carrier. Prior to this, information was transmitted via on/off keying of a continuous wave transmitter using Morse code or some equivalent.
There are several methods for generating AM in a transmitter.
1. Low level modulation. The modulation is developed in the first stage RF section, then amplified by subsequent stages to full power. Simple and easy to implement, especially for mobile transmitters and SSB installations. Disadvantages come from the need for linear amplification through all the stages requiring class A or AB amplifiers and does not reproduce wide band AM well.
 Grid Modulated AM transmitter 2. Doherty modulation. William Doherty came up with an ingenious way to use a low level linear modulator with good to excellent efficiency. Under full carrier, no modulation conditions, the carrier tube is generating the RF carrier and the peak tube is mostly cut off (very little current). When modulation is applied, the peak tube then begins to conduct, the output of this tube is combined with the output of the carrier tube through a 90° LC network, which is the same as 1/4 wave length transmission line. The effect of this is to lower the output impedance, thus allowing the carrier tube to modulate 100 percent.
Later, Continental Electronics and Jim Weldon somewhat modified this system in their 317C series high power transmitters.
 Continental 317B simplified schematic diagram 3. High level or plate modulation. The RF and Audio sections are developed separately with in the transmitter, then combined in the final stage of the transmitter. Older systems used a modulation transformer. Advantages are all the amplifiers can be run class C or greater, which reduced electrical consumption and power supply requirements. Much higher power levels are achievable with this design. These transmitters also reproduce wide band audio much better than low level modulated units. They are also extremely rugged. Disadvantages are the system requires large audio section and they take up greater area and are not as efficient as later modulation methods.
 Plate Modulated AM transmitter 4. Ampliphase. A phase modulated system developed by RCA where the transmitter developed two RF signals in the final, 135 degrees apart. To modulate the signal, the phase relationship between the carriers is varied, more toward 180 degrees would be a negative peak and more toward 90 degrees a positive peak. These transmitters required less space and where more efficient than traditional plate modulated transmitters. They required careful set up and tune up to reduce distortion and somewhat unfairly earned the name “amplifuzz” from some engineers.
 RCA BTE 20 ampliphase AM excit 5. PDM or PWM. This is also a high level modulation scheme, but with some slight variations. The carrier power level and modulation levels are set by a PDM encoder card. In Harris transmitters, the PDM frequency was 75 KHz. The carrier is set by the amplitude of the PDM wave form, the modulation is determined by the duration of the pulse. PDM transmitters require power supply voltages about twice the voltage of a standard high level plate modulated transmitters. They also require a damper diode to conduct to conduct the B+ voltage to back to the power supply during negative peaks, otherwise the PA voltage will attempt to rise to infinity. I have found the damper diode to be the weak link in a tube type PDM transmitter.
Solid state transmitters also use this design with either MOSFETs or BJT, which are then combined in parallel to generate the required output power. This is most often called “Class E” or something similar. In that system, each pair of modulator MOSFETs has it’s own fast acting damper diode, usually protected by a fuse.
 Harris MWx tube type PDM transmitter 6. Direct Digital Synthesis. This is a patented design from Harris broadcast used in their DX series transmitters. The incoming audio is sampled at either the carrier frequency or 1/2 the carrier frequency depending on where in the band the station falls. The solid state PA modules are then switched on and off at the carrier frequency with the audio levels imposed on the carrier information. The explanation is simple, the application is complex:
 Harris DX series AM transmitter block diagram Of all these transmitters, the Harris DX series are the most efficient from a power input (AC) to power output (RF) perspective. There are several methods of reducing electrical use by reducing carrier power levels during lulls in modulation. The Continental 418E and later series transmitters can reduce carriers up to 6 dB using CCM. Harris and Nautel use similar systems on their DX and XL transmitter respectively. Wheatstone corporate blog has an article: Greener AM transmission Methods that details others.
As far as simplicity, serviceability and rugged design, the high level plate modulated transmitters cannot be beat. Many Amateur Radio operators build these units from scratch using old parts, tubes and other reused equipment readily available, often for free. I have, in fact, donated several 1 KW AM transmitters to ham radio operators over the years.
If I were to design a “transmitter of last resort,” to use in case everything else fails, it would look something like this:
 813 Tube type 250 watt transmitter final  813 Tube type AM transmitter modulator section The disadvantage of that design is it requires a 2KV power supply, which has it’s own set of safety concerns. I might substitute 833s for 813s and use heavier iron in the modulation transformer. That way the transmitter could develop a 500 to 1,000 watt carrier. The great thing about tube transmitters is, given the right output components, they can be tuned into almost any load. They are also easily adaptable for emergency operation into temporary antennas.
By Paul Thurst, on February 25th, 2011
Looks like the AM HD Radio™ juggernaut continues… To sink under it’s own technical faults that is. According to the list the number of AM stations running IBOC in the US is now down to 233 stations from a high water mark of 290 or so. That represents a total of just 4.8% (233 IBOC/4782 Total stations) of all US AM radio stations. On a related note, Bob Savage of WYSL 1040, Rochester, NY has a good idea:
I’ve always said – if you want to see surprising new life in the AM band, s**t-can the stupid irrelevant NRSC pre-emphasis filter and allow stations to run to 15 kHz during daylight hours and 10 kHz nighttime. Mandate C-QUAM in all receiver and receiver devices.
It will sound better than HD, be more robust, and cause far fewer problems. Plus it wouldn’t obsolete a single radio out there, while making a whole bunch of them sound a whole bunch better.
It is so simple in concept, so easy to implement, with almost no expense to AM stations. Again, Mr. Savage:
Most software-based processors have com ports which can be addressed by a remote control system like Sine Systems, so when the power gets reduced at evening pattern change, the bandpass can be changed at the same time…..vice-versa at sunup. No biggie.
For older setups a simple outboard relay and rolloff network could accomplish the same thing. It’s a little more complex but again, not a big deal.
Wow. Facepalm.
Wish somebody had though of that a few years ago, it might have save several million dollars and we’d have a different AM band today.
There are a few shoe horned AM stations around here that might be adversely effected by 15 KHz daytime bandwidth, but those are few and far between. By and large, most stations are spaced correctly where this could really work and work well. It certainly would not generate the chaos that AM HD Radio™ has.
By Paul Thurst, on February 23rd, 2011
WISN 1130 AM has been on the air since 1922, although not always with those call letters. In an interesting twist, the license was granted to the local newspaper, the Wisconsin News and the Milwaukee School of Engineering. Initially, both entities were programming the station, however, by about 1925, the newspaper was responsible for programming and the engineering school was responsible for technical operations.
In 1941, the station increased power from 1,000 watts to 5,000 watts and added night time service. This is a series of pictures from that time period.
 WISN night time allocation study Back in 1941, night time interference was taken seriously. The night time allocation study (on 1150 KHz, WISN’s former frequency) includes co-channel stations in the US, Canada, Cuba and Mexico.
 WISN night time allocation ma The array consisted of four Blaw-Knox self supporting towers in a rectangle. Notice the lack of fencing, warning signs and the like around the towers.
 WISN antenna array From the front of the transmitter building
 WISN transmitter site, 1941 The site looks well designed, no doubt manned during operation, which at the time would likely be 6 am to midnight except under special circumstances. Most of these old transmitter sites had full kitchens, bathrooms, and occasionally a bunk room. The transmitter operators where required to have 1st telephone licenses from the FCC. There is only one manned transmitter site in the US that I know about; Mount Mansfield, VT. There, WCAX, WPTZ, WETK, and VPR have their transmitters.
 WISN RCA BT-5E transmitter, 1941 The WISN RCA BT5E transmitter looks huge for that power level. Back in the day when AM was king, these units were designed to stay on the air, no matter what. I don’t know too much about this model transmitter, but if it is like other RCA/GE models from the same era, it has redundant everything.
 RCA AM antenna monitor Old school antenna monitor. I have never seen one of these in operation, however, as I understand it, the scope was used to compare the phase relationship of each tower against the reference tower.
These pictures are of the WISN 1150 array was it was in 1941. Since then, the station has changed frequencies to 1130 KHz and increased power to 50,000 watts daytime/10,000 watts night time. The daytime array consists of six towers and the night time array has nine towers, all of which are 90 degrees.
Special thanks to John A. for sending these pictures along.
By Paul Thurst, on January 15th, 2011
This is a Hughey Phillips mechanical tower light flasher that has been in service since 1960. Basically it is a motor connected to a cam that rocks a mercury relay back and forth. These were standard technology for tower lights from the 1930′s through about 1970 or a little later. They were very reliable, we still have some with a “pancake motor” in use on some of our towers. They were very robust and immune to lightning damage, RF interference and other problems. The only maintenance that I can think of is lubricating the motor bearings. Eventually, however, they do wear out. Cold weather seems to take its toll, often causing the motor to stop.
 Hughey and Phillips mechanical tower light flashe This particular unit is mounted inside the tuning house for the far tower (north tower) at the WGHQ antenna array. It has finally reached the end of it’s existence; the motor bearings are shot and it has gotten stuck in both the on and off position this year causing the FAA to be notified of the malfunction.
 WGHQ 920 Khz Kingston, NY antenna array Today, I am replacing it with a solid state flasher (SSAC B-KON FS155-30RF). Solid state flasher units have been known to malfunction in high RF fields, such as AM towers. To cure that, the manufacture has built in 0.01 uf bypass capacitors, hence the “RF” suffix. Older units did not have the built in bypass caps, so external 0.1 uf bypass capacitors were normally installed on units mounted to AM towers. While I was working on this, I turned the transmitter down to 500 watts, no need getting any RF burns.
Naturally, this has to happen after there is two feet of snow on the ground. Also, it should be noted that this is the furthest tower away from the transmitter building. Now where did I put those snow shoes? Never mind, it has been very cold and the ground is frozen solid, I’ll take the truck… This is good because I will have all the tools, drills, nuts and bolts without having to walk back and forth several times in the snow.
 Hughey Phillips mechanical beacon flasher I removed the motor and mercury filled relay. I’ll have to figure out how to dispose of the relay. I then drilled a mounting hole through the base of the old flasher housing and bolted the solid state relay to it. This is required because the solid state relay needs a pretty good heat sink.
 SSAC B-KON tower light flasher Turn everything back on and: Ta-da! All works normally, tower beacon is flashing away up there. Time to leave.
 Truck stuck in swamp Pull forward about 2 feet to turn around and CRUNCH! The truck goes through the ice of a hidden stream. Any attempt to move only makes it worse:
 Truck rear burried to axle Put in a phone call to the one guy I know that can get me out. About an hour later he shows up with chains, a shovel and a come-a-long. We attach the come-a-long to the fence support post and pull the truck out backwards 1/2 inch at a time. It took us about an hour and a half to get it all the way out so I could drive it back across the field. I’d have taken some pictures, but my guy; he was a little grumpy.
I won’t do that again.
Still, I did the job I came to do, so it was a good day after all.
By Paul Thurst, on January 6th, 2011
Another one of those things that might work, could possibly provide some service to travelers, but often falls short. Very similar to EAS, the government officials nominally in charge of the system have no idea of broadcasting. The Rhode Island TIS on I-95 had an endless loop proclaiming “This is the Rhode Island Department of Transportation Travelers Information System.” While that was welcome information, in a snow storm, something about road conditions or perhaps the weather forecast might have been more appreciated. According to the FCC website, that was WPTJ441 on 1630 KHz with a power of 10 watts. The transmitter is located at the I-95 rest area on the Rhode Island/Connecticut state boarder.
The FCC is seeking comments on the current TIS in NPRM 10-203:
We solicit comment on whether to modify the existing rules governing the licensing and operation of Travelers’ Information Stations (TIS) to expand the scope of permitted operations. Since the inception of TIS in 1977, the Commission has authorized TIS operations to permit Public Safety Pool eligible entities to transmit noncommercial travel-related information to motorists on a localized basis. However, certain parties and licensees have sought to expand the scope of TIS operations in order to transmit more general alerts and public safety-related information to the public, including non-motorists. By initiating this proceeding, we grant petitions for rulemaking filed by Highway Information Systems, Inc. (HIS) and the American Associations of State Highway and Transportation Officials (AASHTO) requesting the commencement of a proceeding to amend the TIS rules. We deny the petition for declaratory ruling regarding TIS filed by the American Association of Information Radio Operators (AAIRO), but incorporate the issues raised in AAIRO’s petition into this rulemaking proceeding
The TIS as specified in the 1977 RO docket 20509 FCC 77-414 is outdated for the most part. The current rules, 47 CFR 90.242 give pretty specific requirements including things TIS stations are not allowed to retransmit, like NOAA weather forecasts. In one section of the NPRM, it is noted with the currently specified facilities, the average vehicle traveling the speed limit will transit the TIS coverage area in 90 to 120 seconds. For the above station, I found it was listenable for about 5 minutes, from the time I saw the sign next to the transmitter until it faded out, which works out to be about five and a half miles.
One alternate possibility is something like a carrier current station or a leaky coax setup buried in the median. Something like that might have more reliable coverage along the highway without blanketing the general area with a broadcast signal.
Again, in Rhode Island, drivers have the option of calling 511 on their cellphone, provided it is a hands free cellphone. My only comment about that is when there is a major traffic jam and everyone is dialing 511 to get information, the cellular network will get congested quickly. Having a broadcast outlet for people to listen to on their car radios would be a more effective way to transmit information to the masses. As to what information a driver might want; here are a few sugestions:
- Up to date traffic information including alternate routing instructions in case of a major tie up.
- Up to date weather information including roadway surface conditions and possible lodging locations
- Up to date construction project information including alternate routing instructions for both car and truck traffic
- When there are no traveling issues, other general information about rest stops, hospitals, tourist attractions, etc.
The most important thing is the information be up to date and relevant to a traveler.
By Paul Thurst, on November 30th, 2010
At the place of my former employment, there is an issue with AM reception. The building is full of old, electrically noisy fluorescent light balasts, computers, mercury vapor parking lot lights, and every other electrical noise generator under the sun. The second issue is that one of the EAS monitor assignments for two FM class B stations is WABC in NYC. Under normal conditions, WABC puts a fine signal into the area. Listening to it is not problem at my house, in the car and what not. However, at the studio the station is audible, but terribly noisy. Every time one of those FM stations ran a required monthly EAS test originated from WABC, it was full of static and just sounded bad on the air.
The state EAS folks were inflexible as to the monitoring assignment. ”WABC is the PEP station for NY. You should have plenty of signal from WABC at your location,” said they.
At one time, the studio had an active loop antenna (LP-1A) from Belar, which worked, but also seemed to amplify the noise. I decided that the best thing to do was go big and ditch the preamp. I made a diamond shaped receiving loop on two pieces of two by four by eight foot lumber. I wound four turns of #14 stranded wire around this frame and made a 4:1 balun to feed the unbalanced 75 ohm RG-6 coax.
That cured the noise problems and for eight years, WABC sounded pretty good on the EAS monitor.
Fast forward to about a week ago. The roof at the studio building was being redone and all the monitor antennas had to be removed from the roof. The homemade loop was not in good shape. The balun box was full of water, the lumber was cracking and falling apart, the insulation was degraded by UV exposure, etc. My boss asked, “how much to make a new one?” So I said something like forty dollars and a couple of hours. He then said, “Make it so we don’t have to ever make another one.”
Music to my ears. I started by checking my assumptions. I made a model and ran NEC to see what the electrical characteristics for that size loop were on 770 KHz. It came out better than I thought, about 1 ohm resistance and 282 ohms inductive reactance. Fooling around a little more showed that roughly 1.3 uH inductance and 720 pF capacitance in a L network would bring this inline for a 50 ohm feed point. Since this is a receive only antenna, that is not a prime consideration. I am more concerned with noise reduction and maintaining at least the bi-directional quality of a loop antenna.
 NEC 2 model AM receive loop Then, I decided to get fancy. What if the capacitance was put on the end of the loop to ground instead of the feed point. That, in effect should make the loop directional off of the unterminated side. Driving the feed point with a 9:1 balun would also bring up the inductance on the feed point. Finally, grounding the whole thing with a separate ground lead might also get rid of some noise.
The final configuration looks something like this, which is essentially a top loaded vertical:
 Low noise AM loop antenna Now to build it.
Once again, I felt that a non-conductive support was needed, so I used two by four by eight foot lumber, but this time I painted them with oil based paint. The side length worked out to be 5.7 feet per side, or 23 feet per turn for a total of 92 feet of wire.
I purchased 100 feet of PV (photovoltaic) wire (Alpha wire PV-1400), which is UV, heat, moisture resistant and designed to last for 30 years in outdoor, exposed environments.
For the balun box, I used a metal outdoor electrical box with a metal cover. I put a ground wire jumper between the box cover and the ground common to maintain shielding. I used a water tight bushing to feed the antenna wires and the ground wire into the box. I drilled a 3/8 hole for a type F chassis connector. Everything was given a little extra water proofing with some silicone based (RTV) sealant on all threaded junctions.
The spreaders for the wire windings are UV resistant 1 inch PVC conduit. I drilled four holes, three inches apart in each spreader to run the loop wires through.
The balun is 7 trifiler turn of 24 AWG copper wire on an FT-43-102 toroid core. Trifiler means three wires twisted together before winding the toroid core.
I used all stainless steal screws and mounting hardware.
The loop is terminated with a 500 pF, 500 volt ceramic capacitor to ground. Once in place, I am going to experiment with this by jumping it out of the circuit to see what effect it has on noise and signal strength. I may also try replacing it with a 200 ohm resistor and or a 1000 pF capacitor.
The assembly was pretty easy, although time consuming. My four year old son helped me paint the wood and string the wires through the spreaders.
I soldered all wire connections with 5% silver bearing solder.
When the whole thing was assembled, I tested it out with my Drake R8 receiver. It performs much as expected, low noise, directional away from the terminated wire loop. It does not appear to be too narrow banded either, as the stations on the high end of the dial were also received with good signal strength.
Next was loading it on the pickup truck, driving it in and mounting it on the studio building. I got some funny looks from my fellow travelers, then again, I usually do.
For the ground, I purchased an eight foot copper clad grounding rod and pounded it into the ground at the corner of the building. This area is always wet as it is the lowest area around the building and all the gutters drain there. This is not be best RF ground, but for the purposes of this antenna, it should work fine. I used about 28 feet of left over #12 stranded wire from the ground rod up to the balun box and connected it to the common ground point inside the box.
The frame itself is mounted on a standard wall mount antenna pole. Stainless steel clamps hold the wood frame to the pole.
Once it was installed, I used my Kenwood R-2000 receiver to find the best mounting azimuth and locked everything down. I also put a toroid on the RG-6 coax coming up from the rack room to keep any shield noise from getting into the antenna.
 AM receive loop PVC wire spacers  AM receive loop wood frame  AM receive loop balun transformer The tuning capacitor is in there too, behind one of the loop wires.
 AM loop antenna installed on roof Antenna installed. I did try substituting the 500 pF capacitor with a 220 resistor. The signal strength came up somewhat, but the noise increased more, therefore the capacitor is a good termination for this antenna.
With this antenna, the signal from WABC is nice and clean and sound good on the FM station when a monthly EAS test is retransmitted.
By Paul Thurst, on November 22nd, 2010
I have received an e-mail from occasional reader John, who comments that many of the Windy City AM’s have turned their buzz saws off. I note myself today, the same can be said for many of the NYC AM’s. WABC has had their’s IBOC turned off for quite some time. The latest to turn off is WNYC on 820 KHz. Several people have noted the loss of noise on their signal this morning.
According to Ibiquity’s own website, only six AM stations in the NYC market are currently using IBOC.
What does this mean?
Could it be that management is finally realizing that the cure is worse than the disease? The disease being alleged poor audio quality, and the cure being IBOC itself.
|
Axiom
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution
Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin
...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner
Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19
|
Heard in the clear