Work continues on rebuilding the North Adams tower after the collapse of March 2014. Over last winter, a new tower was erected. This is a fairly substantial tower.
New North Adams tower on ground
North Adams new tower erected
In the interim, a new Shively 6810 four bay half wave spaced antenna was ordered. This antenna will be combined for two stations, WUPE-FM and WNNI using a Shively 2630-2-06 branched combiner. The 70 foot utility pole next to the building will be retained as backup facility for both stations. The Shively Antenna went up in stages.
New WUPE-FM and WNNI Shively 6810 antenna
Tower climbers rigging tower for new antenna
Prescott Tower from Rutland Vermont was on site to do the tower work. They were the primary contractor for installing the new tower and did a really nice job of it.
New North Adams tower ice bridges to various shelters
Hanging the top two bays of new antenna
Lift of bottom two bays and first tuning section
Securing bottom section and bolting bays together
After that, there was twenty feet of rigid line, another tuning section, then the 1 5/8 inch helax into the transmitter room. The antenna was tuned and the load looks very good. We are waiting for the electrician to finish wiring up the new racks and we will move both stations into their new home.
Hurricane season is here. This time of year makes me fondly remember hurricanes of the past and the things we had to do to get stations back on the air; walking a mile down a sandy spit of land, wading through swamp water to get to the transmitter shack, being threatened with arrest by the Connecticut National Guard, blow drying RF modules with a hair dryer, sleeping in a camper for a week… Ahhhh, good times, great times!
The one thing that I did learn, if the disaster is big enough, expect none of the normal services to be functioning. That includes things like gas stations, fuel delivery, grocery stores, restaurants, hotels, UPS, roads, bridges, telephone service, internet service, etc.
It is not a far fetched scenario for the main FM transmitter site to be out of commission and will not be available or accessible for some prolonged period of time. There might also be mitigating circumstances such as catastrophic tower failure, destruction of transmitter building, flooding, or other major infrastructure disruptions. In those situations, calling the broadcast supply vendor of choice for a replacement might not be an option.
It has happened before…
All of these things got me to thinking about how to fabricate a reliable FM broadcast antenna from simple materials available on hand. The FCC allows for temporary operation with an emergency antenna in part 73.1680, which reads:
(a) An emergency antenna is one that is erected for temporary use after the authorized main and auxiliary antennas are damaged and cannot be used.
(b) Prior authority from the FCC is not required by licensees and permittees to erect and commence operations using an emergency antenna to restore program service to the public. However, an informal letter request to continue operation with the emergency antenna must be made within 24 hours to the FCC in Washington, DC, Attention: Audio Division (radio) or Video Division (television), Media Bureau, within 24 hours after commencement of its use. The request is to include a description of the damage to the authorized antenna, a description of the emergency antenna, and the station operating power with the emergency antenna.
(1) AM stations. AM stations may use a horizontal or vertical wire or a nondirectional vertical element of a directional antenna as an emergency antenna. AM stations using an emergency nondirectional antenna or a horizontal or vertical wire pursuant to this section, in lieu or authorized directional facilities, shall operate with power reduced to 25% or less of the nominal licensed power, or, a higher power, not exceeding licensed power, while insuring that the radiated filed strength does not exceed that authorized in any given azimuth for the corresponding hours of directional operation.
(2) FM, TV and Class A TV stations. FM, TV and Class A TV stations may erect any suitable radiator, or use operable sections of the authorized antenna(s) as an emergency antenna.
(c) The FCC may prescribe the output power, radiation limits, or other operating conditions when using an emergency antenna, and emergency antenna authorizations may be modified or terminated in the event harmful interference is caused to other stations or services by the use of an emergency antenna.
In this situation, making a circularly polarized antenna would be overly complicated, so either a horizontally or vertically polarized antenna would be the most likely scenario. There are a few antenna types that readily lend themselves to field expedient fabrication.
These are, in no particular order:
Of these, the 1/2 wave wire dipole is the easiest to construct. Cut two wires, length (in feet) determined by the formula 234/Frequency (Mhz). Attach one wire to the center conductor and one to the shield, stretch to the wires out and tune for minimum SWR by cutting or adding small lengths to the ends. The total length for such an antenna would be approximately five feet and it could be mounted horizontally or vertically. The issue with a wire dipole would be bandwidth and power handling capability.
A 1/2 wave dipole made from tubing would have better bandwidth and power handling, but tubing is a little harder to work with when it comes to tuning the antenna.
Frankly, if one is going to go through the trouble of using tubing to create an emergency antenna, the the J-Pole (end fed antenna with a 1/4 wave matching section) is probably the best. This antenna is easier to tune, does not need to work against a ground plain, has good bandwidth and a low take off angle, meaning more power is radiated out toward the horizon, giving it a good deal of gain over both a ground plane or dipole antenna. Additionally, when using standard RG-8, RG-214, LMR-400 or other similar transmission line, a well matched antenna might be able to accept about 1 KW of input power, which would net approximately 4.4 KW ERP. Not an insignificant sum, especially in an emergency situation.
Vertical radiation pattern for J-pole (1/2 wave end fed) antenna
1/4 wave ground plane vertical radiation pattern
There are many J-Pole antenna calculators available on line, but many of them include a 20 inch or so section of tubing below the tuning stub that can be electrically coupled to the supporting structure. This configuration defeats the main advantage of the antenna, creating a good deal of upward radiation. It is a better idea to use a non-conductive support piece and keep any conductive materials at least 1/2 wave length or greater from the radiating portion of the antenna.
The basic j-pole antenna looks like this:
J Pole (1/2 wave vertical antenna) diagram
The radiating part of the antenna starts above the tuning stub. Basically, the 1/4 wave stub is shorted at the bottom, the feed point is adjusted away from the shorted end until a 50 ohm impedance point is found. The center conductor of the coax is attached to the 3/4 wavelength section, while the shield is connected to the stub. The critical distances are the tuning stub length and the distance of the feed point from the shorting section. I created an excel spreadsheet (.xls) that can be used to create all the lengths required to fabricate one of these antennas. That spreadsheet can be had here: J Pole Calculator
Having a few moments of time to spare, I thought it would be fun to build one of these and put the analyzer to it. I think testing things in the real world is a good exercise and I always enjoy working with antennas anyway. Looking in the basement, I found some 3/4 inch copper tubing, a tee, an elbow and a few end caps. The complete list of parts is thus:
|¾ copper tubing
||78-96 inches (196-244 cm) (frequency dependent)
|¾ copper tubing
||26-32 inches (66-82 cm) (frequency dependent)
|¾ copper tubing
||2.5-3 inches (6.35-7.62 cm) (frequency dependent)
||Tuning stub short
|¾ copper tubing
||2 inches (5.08 cm)
||Mounting section, bottom of T to MIP threaded adaptor
|¾ copper T section
||T section for joining main section to tuning stub
|¾ copper 90 elbow
|¾ copper end cap
||End cap on tubing
|¾ to 1 inch copper MIP threaded adaptor
|1 inch PVC FPT threaded adaptor
||Insulating mounting connection
|1 inch PVC
||Approximately 20-25 inches (50-65 cm)
||Insulating mounting material
|1 inch stainless steel hose clamps
||Attaching the coax to the antenna feed point
|RG-8, RG-214, LMR-400 or other transmission line
||As needed, including 5-6 turns, six inches in diameter to form RF choke at feedpoint
||RF choke needed to keep RF off of coax shield
One important detail to remember when using the above spreadsheet, the measurements are to the closest side and not the center. Thus, if something measures 2.5 inches, it is metal to metal. Some basic soldering skills are required, but assembly is relatively straight forward. In a pinch, almost any conductive material could be used including aluminum, brass, steel, EMT, rigid conduit, or even iron pipe.
Parts cut to size for J-pole antenna on 87.9 MHz
J-pole antenna assembled
J-pole antenna on the antenna testing range
I made this particular J-pole antenna on 87.9 MHz because I didn’t feel like chopping up all my 3/4 inch tubing. Cutting and soldering the tubing took about a half an hour. Designing and fabricating the feed point system another half an hour. I’ll throw another hour in for rounding up the parts, tools, etc. Thus, entire antenna was constructed in about two hours. I used my AIM 4170D to find the proper feed point. If I were going to actually use this antenna, it would then be a matter of finding a mounting location and running the transmission line.
J-pole antenna analysis results
Actually, I was less than happy with this. While the antenna is nice and broad across several channels, there is 16 ohms inductive reactance that is impossible to get rid of. That gives an SWR of 1.4:1, which is not great. With that kind of load, I would be reluctant to run more than a couple of hundred watts into this antenna. The interesting thing is, that graph is the first one, with everything set as calculated in the spreadsheet. After that, I could make the impedance and reactance worse, but not better.
Still, in a pinch, I would use this antenna until something better could be found.
As promised, a picture of the feed point:
J-pole feed point connections
The hose clamps are not optimum, I am sure a better way to attach the feed line to the antenna can be fabricated, but again, I was thinking of an emergency situation and the parts which may be available from local sources.
Whilst working in the generator room at WFLY, I found this bit of treasure stashed on an overhead shelf:
General Electric BY-4-C FM circular broadcast antenna, ca 1948
That is a very old FM broadcast antenna from 1947-48. It must have been intended as a spare antenna in case the main antenna had a problem. It was never needed, so it remains in its original shipping crate. I would think that these were rather well made, since the original main antenna was in service from 1948 until 1970 or so, when it was replaced with a Shively 6710.
General Electric BY-4-C antenna element
The entire antenna is intact including the interbay lines, power divider T’s and tuning section. Of course, it is of little use to the radio station today, as it is horizontally polarized. Perhaps some museum somewhere? I don’t know, it would be kind of neat to put it all together and use it as an exhibit.
The news is out; this is for the new “WFAS-FM” which is actually W232AL retransmitting the WPLJ HD-2 channel. What do they call translators these days… Metro stations? Something like that. Anyway, quite a bit of work went into getting this off the ground before the start of Labor Day weekend and here it is!
We are currently working on a project that involves installing a Shively 6810 FM antenna. Since few people get to see these things up close, I thought I would post a few pictures.
This particular antenna is a four bay, half wave spaced directional antenna. It is going to be side mounted on a 430 foot tower. To do this, we had to lower the AM skirt wires by about fifteen feet and retune the AM antenna.
This Shively antenna came in seventeen boxes with sixty four pages of assembly instructions. There are many parts and they need to be assembled in the order specified, otherwise things get in the way. We found that Shively provided many extra bolts, washers, O rings, etc because things get lost. Also, all of the parasitic locations and bay orientations were clearly marked. One thing that the tower crew said; always check the allen screws and other hardware on the elements before installing the RADOMES.
Shively 6810 installing elements
Since this is a half wave antenna, the radiating elements are 180 degrees out of phase, bay to bay.
Shively 6810 mounting brackets
Stainless steel tower leg mounting brackets.
Assembled element with RADOME. This is the top bay with the gas pressure release valve
Shively 6810 top bays staged for hoist
We hoisted two bays at a time. The top bays are ready to go up.
Shively 6810 top two bays lift
The bottom two bays were hoisted next.
Shively 6810 four bays installed
This is the antenna installed, less the tuning section and parasitic elements. It is tilted off axis from the tower by design due to its highly directional nature.
The transmission line was installed and we swept the antenna. I will snap a few final pictures once the transmitter is installed, which will happen tomorrow.
Updated Pictures: Here are a few pictures of the finalized installation:
W232AL antenna, new installation on WFAS AM tower
The fully installed antenna, tuning unit and transmission line. We did some program testing, made sure the audio sounded good, then the station was signed on. We also had to lower the AM station’s skirt a few feet and retune the ATU. Actually, the ATU needed to be reconfigured because the shut leg had been disconnected and there was a capacitor added to the circuit after the base current meter. All of that was fixed, along with a few other things…
W232AL transmitter, a BW Broadcast TX300 V2
The W232AL transmitter is a BW Broadcast TX300 V2. These little transmitter are packed with features like a web interface, on board audio processing, etc. They are pretty neat.
It’s Friday, time to go home!
The tower crew from Northeast Towers did a great job, as they always do.
Shively 6710-1 FM antenna
Perhaps that is one Shively Antenna that you haven’t heard of. They were an odd-ball combination of a horizontally polarized antenna with an adjustable vertical element. This design allowed the station to adjust the ratio of horizontal to vertical power from a range of 1:1 to about 4:1 (H:V). Why would this be a desirable feature?
Back in the early days of FM broadcasting, almost all stations had horizontally polarized antennas. This system worked remarkably well, stations could broadcast at moderate power levels over fairly long, line of sight (or mostly line of sight) paths. Most FM receivers were stationary units installed in people’s homes often with outdoor antennas.
It was not until the late 1960’s and early 1970’s that FM radio receivers became a stock option in most low and mid cost automobiles. It was then that a slight problem with FM broadcasting was discovered; car antennas are vertically polarized. People driving around in their new machines found that the FM reception was not all that great. Stations began adding a vertical component to their signal to help improve the mobile reception situation.
I found this Shively Brochure in a file cabinet drawer at the WFLY transmitter site. This model antenna was ordered and installed by that station in 1970. It had a 3:1 horizontal to vertical ratio. Why not install a fully circularly polarized antenna? Because often that necessitated installing a new, more powerful transmitter. Every watt of power taken from the horizontal plane and added to the vertical plane reduced the ERP by that much and had to be made up with more transmitter power output. Often times, the ratio of H:V power would be adjusted to take up whatever headroom there was in the transmitter and the station would run that way until the next transmitter replacement cycle.
I found the remains of this antenna in the woods, north east of the tower.
Shively 6710 antenna section
This section looks pretty well destroyed. It is probably better to dispose of these type things by scraping, rather than dumping them in the woods. While there is not a lot of scrap value to this unit, it can become an attractive nuisance copper thieves and other vandals if it is left laying about.
It is a strange looking piece of kit, a sort of make do until the situation could be fully rectified. I think this antenna was in service until 1986 or 87 when it was replaced with a circularly polarized ERI.
FCC rules stipulate that when a station is operating at a variance from its licensed parameters for more than 10 days, Special Temporary Authority (STA) is required. The reasons for requesting an STA are varied but could include things like:
- Damaged transmission equipment
- Loss of transmitter site or building use
- Loss of tower
- Facilities upgrade or renovation
- Natural disaster
The loss of transmission tower at WUPE-FM falls into one of those broad categories. Thus, we have filed a STA with the FCC for temporary transmission facilities while a new tower is being constructed. Since the old tower is completely lost, we specified a new tower location, new height above average terrain (HAAT), new ERP and environmental certification. To gather that information, several steps were needed:
- Obtain new tower location. This was done with a GPS receiver and verified on itouchmap.com. Once the NAD83 position was obtained, it needed to be converted to NAD27 for the FCC filing. The FCC has a conversion tool on their website.
- HAAT calculation is fairly simple, use the HAAT calculator tool on the FCC website. For this, the antenna radiation center height Above Mean Sea Level (AMSL) is needed. Using a topographical map, find the ground level AMSL, convert it to meters, then add the radiation center height above ground level (AGL).
- The Effective Radiated Power (ERP) calculation is also simple; Transmitter Power Output (TPO) minus system losses (transmission line and antenna gain). It is easiest to do this in dBm, e.g. convert the TPO from Watts to dBm, then add or subtract the gain or losses in dB, convert the final product back to Watts.
- The environmental statement is slightly more tricky. Basically, the filer is certifying that the STA complies with all environmental regulations including OET-65 (RF exposure limits). Since the temporary antenna is significantly lower than the original, some investigation is required. For this, there are two methods to demonstrate compliance; ground measurements with a NARDA meter, or RFR worksheets which are a part of the broadcast station renewal form, FCC-303s.
I have taken the RF worksheet sections out of the 303s and separated them into the FM RF Worksheet and the AM RF Worksheet. These worksheets are not effective for large tower farm type sites where there are too many variables and RF contributors to be accounted for. The calculations on the worksheets are not conclusive, however, if the facility in question falls under the limits, it is generally accepted as being in compliance. Taking ground measurements with a NARDA meter is the definitive method for determining RFR compliance. Since this is a relatively simple site, the worksheet calculations should be sufficient.
The worksheet calculations show that the RFR is with in both the controlled occupations limits and the uncontrolled general population limits.
WUPE-FM temporary antenna RFR worksheet
The position of the new temporary pole verified on itouchmap.com:
It is never good to be operating at a varience from licensed parameters without notification of the FCC. Such things could lead to fine or other problems for the broadcaster.
Zorch is a term used to describe an over voltage or over current condition that usually leads to catastrophic failure, e.g. the power supply was zorched by lightning. There is also a quality to radio signals that defy and exceed theoretical definitions for service contours or power density. That is quality defined as:
Zorch (adj): The ability of an RF signal to be received in unlikely locations; outside of predicted service contour, in steel structures, underground facilities, tunnels, etc.
It brings to mind the saying, “antennas are not amplifiers and amplifiers are not antennas.”
ERI circularly polarized 2 bay antenna
During the earlier stages of FM broadcasting, there was a notion that costs could be reduced by increasing antenna gain and reducing transmitter size. While theoretically, ERP (Effective Radiated Power) is ERP, broadcasters soon learned that high gain antenna, low TPO (Transmitter Power Output) installations lacked building penetration and had other reception issues. Realizing that there is a trade off between antenna bays, transmitter power output especially in difficult reception areas, a great debate occurred and continues on what the optimal system is. The answer is, it depends on the receiving environment.
Where this technical detail can be really important is with lower powered FM stations; Class A and LPFMs to be exact. They are already battling against bigger stations that have tens or even hundreds of times more power. Certainly an LP-100 station has it’s work cut out for it. The choice of antenna is perhaps one of the most important technical decisions to be made. Choosing the right balance of antenna type, antenna gain, antenna height and transmitter power output can greatly influence reception reliability and thus coverage area.
A good study of this quality can be had by looking at various LPFM installations:
|Station ERP (watts)
||Antenna Gain (power)
||Coefficient of Zorch
||1 bay vertical
||1 bay circular
||2 bay vertical full
||2 bay vertical half
||2 bay circular full
||2 bay circular half
||3 bay circular full
||3 bay circular half
*Includes 100 feet of 1/2 inch foam transmission line, Andrew LDF4-50A, loss of 0.661 dB at 100 MHz, or 0.859 power gain.
Stations should try to get the transmitting antenna as high up as permitted without reducing ERP. In other words, the FCC allows 100 watts ERP with 98 feet Height Above Average Terrain (HAAT) radiation center in their current LPFM rules. Being lower in height will reduce the coverage area. Going over 98 feet HAAT will cause the station’s power to be reduced, which will lower the coefficient of zorch accordingly. Therefore, getting as close to 98 feet HAAT, which is different than 98 feet above ground level in many places, will net the best performance.
If a singular polarization (horizontal or vertical) is desired, vertical polarization should be chosen, as most mobile reception is by a vertical whip antenna. For best reception performance, a circularly polarized antenna will work best, as receiver antenna orientation will not effect the signal reception. A circularly polarized antenna has better building penetration and multi-path characteristics. The FM broadcast circularly polarized antenna in not a true circularly polarized antenna, it is actually unpolarized.
The use of a multi-bay antenna has the effect of focusing the RF radiation outward, perpendicular to the element stack, thus limiting the radiation directly up or down from the antenna. This is more pronounced with one half wave spaced antennas, which may be an environmental consideration in heavily populated areas.
Thus, the best coefficient of zorch for an LPFM station would be a circularly polarized, 1/2 wave spaced, 2 bay antenna. This antenna would have some gain over a single bay antenna, take up less room on a tower than a full wave spaced antenna, offer good RF protection performance for the general public living and working under the antenna, reduce wasted upward radiation and offer good building penetration for the ERP. It would require a slightly larger transmitter and more electricity, but that trade off is well worth the effort.
This was in the back room of one of the radio stations we do work for:
ERI LPX2E FM antenna with RADOMES on the ground
It is a burned out ERI LPX2E antenna. The manager was complaining that it took up too much space and he didn’t know what to do with it. Could I get rid of it? Sure, no problem. I could at least cut it up and scrap it.
When I first looked at it, is seemed complete and undamaged, however, upon further examination it seemed that some of the inter bay line had over heated and one of the tuning sections that goes from the power divider out to the bay was missing. Therefore, I took it apart and separated the copper from the brass. Most of the antenna is made from yellow brass, due to it’s hardness. The inner line sections are copper and the mounting hardware is all stainless steel. I will perhaps break even time wise, but it is one of those projects that can be done on my time in between other paid work, so it will be fine.
ERI rototiller antenna bay
I am going to keep one bay intact with the RADOMES on as some sort of modern art project. My daughter thinks we should install it in the yard as a part of a fountain-fish pond-bird bath contraption. The idea is to mount the bay facing up as in the picture above and run a hose up the inside of the transmission line to the T section. A hole will be drilled there and some type of fountain head installed to spray water up over the RADOME. The system will be run by a solar powered pond pump. I’ll have to take pictures when it is done.