I was watching the tower crew measure the guy tensions on this tower the other day:
AM tower
The preferred method is to use a pulling force to move the guy wire slightly off of the turnbuckle or hairpin bolt then measure that force. Hydraulic rams with a pressure gauge are used more often then not.
hydraulic ram, showing 7,000 lbs pulling force
These guy wires are called “Bridge Strand,” the bottom wire is is 7/8 and the top is 1 1/8 inches. Normally, these are tensioned to around 5 – 10% of their breaking strength at 60 degrees F. In this case, it was slightly colder so the tensions are a little bit higher. If it would have been warmer, then they would be a few percentage points lower.
A couple of videos for your viewing enjoyment:
Lower guy tension 7,400 lbs, Upper guy wire tension 10,500 lbs. Both are well within their ranges.
Occasional reader Scott asked for a picture of the inside of a BE AM output tuning network. I figured it might be helpful to make a short post about it.
These things are pretty simple; a T network with a capacitive leg to ground.
BE AM Output tuning network
This particular unit is for 1230 KHz. I believe the capacitor is frequency determined and they may also use larger inductors for lower frequencies.
BE AM output tuning network schematic
The inductors are Kintronic LV-15-20 (15uH 20 amp) and the capacitor is 0.0018 uF CDE 6KV 5.6 amp.
The issue with this particular unit is dirt. The inductors have round metal plates that roll along the inductor coil to make the variable inductor tap. Dirt has accumulated on the coil turns and on the inside of the plates. This, in turn, causes arcing anytime the Tune or Load controls are moved. A thorough cleaning should take care of the problem.
Working on another old AM station, this one is a simple Class C one tower on 1230 KHz.
Broadcast Electronics AM Output Tuning Network
The main problem today was this BE AM output network unit between the BE AM1A and the ATU. This site has had some dirt difficulties over the years and the internal parts of this tuning unit arc at full power. I attempted to drive the ATU directly with the transmitter, which was a no-go.
Gates Radio 1 KW AM ATU, circa 1947
I took a look at the ATU, which is a pretty standard Gates 1 KW ATU from the late forties or early fifties. I have seen perhaps dozens of these things.
My first thought was that over the years, likely due to changes in the ground system, the base impedance has shifted away from its licensed values. However, a quick measurement of the base impedance shows it to be exactly at the licensed value, 17.3 ohms. The tower is 67 degrees tall so the impedance value is right in the theoretical norm.
I measured the input to the ATU, which showed 38 ohms with about 7 ohms of capacitive reactance. I can only surmise that it has always been this way. The transmitter in use before the BE AM1A was a Harris/Gates Radio BC-1G. That model transmitter will drive anything including an open transmission line.
Having the bridge on hand, I decided to retune the ATU for a better match. I put the bridge on the input terminals of the ATU and set it to 50 j0. Using the remote control, I turned the transmitter off and on while making small adjustments to the output strap on the coil until the resistance was 49 ohms with zero reactance. I would have gotten it to 50 ohms, but the strap on the output side of the coil would not stretch far enough to reach the proper spot on the coil.
Now the transmitter will run into the ATU directly at full power with about three watts reflected. The BE AM output matching network unit has been removed for cleaning and repairs. I will reinstall it once those repairs are completed.
I am currently finishing an interesting project involving putting up two translators on a diplexed AM tower which also holds a mobile phone/data tenant as well. All-in-all, this seems to be a very efficient use of vertical real estate.
WMML WENU tower, Glens Falls, NY
The AM stations are WMML and WENU in Glens Falls, NY. The AM stations are diplexed using a Phasetek diplexor/ATU.
Diagram showing WENU/WMML tower with W250CC/W245DA antenna installedDiplexor diagram, WENU/WMML Glens Falls, NY
The translators are W250CC and W245DA which are using a NICOM BKG-77/2 two bay 3/4 wave spaced antenna mounted at 53 meters AGL. The translators use a Shively 2640-04/2 filter/diplexor which is a broadband input port in addition to the translator input ports. Since these translator signals are only 1 MHz apart, the higher-power Shively filter was installed because it has better rejection characteristics. The broadband input port allows the NICOM antenna to be used as a backup for any of the three FM stations; WKBE 107.1, WNYQ 101.7, or WFFG 100.3. Two transmitter sites for those stations are mountaintop locations which are very difficult to get to in the wintertime. Having a backup site available takes some of the pressure off during storms or other emergencies.
Shively 2640 -04/2 filter for W250CC and W245DA
The NICOM FM antenna was mounted on the tower when W250CC went on the air in October 2016. When it was installed, the base impedances for both AM stations were measured. For some reason, WENU 1410 KHz seems to be more sensitive to any changes on the tower, thus the WENU ATU needed a slight touch-up. When working on diplexed AM systems, it is also important to make sure that both trap filters, which are parallel resonant LC circuits, are tuned for maximum rejection of the other signal. During this particular installation, nothing was added to the tower and no change in the base impedance for either station was noted.
Shively Filter, connected to transmitters and antenna
As a condition of the construction permit, measurement of spurious emissions of all stations sharing the common antenna needed to be completed to ensure compliance with FCC 73.317(b) and 73.317(d). I made careful measurements of the potential intermod products between the two translator frequencies. This measurement was completed with my TTI PSA6005 spectrum analyzer.
The primary concern here is mixing products between the two transmitters. Both transmitters are BW TXT-600 with low pass filters before the output connector. There are three frequencies of interest;
(F1 – F2) + F1 or (97.9 MHz – 96.9 MHz ) + 97.9 MHz = 98.9 MHz
That, plus harmonic measurements out to seven or eight harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out-of-band emissions standards. Being that this site has LTE carriers, it is very important to measure the harmonics in those bands. Mobile data systems often use receiver pre-amps, which can amplify harmonics from the FM band and make them look out of compliance. Having a base set of readings to fall back on is always the best course in case the “out of tolerance” condition gets reported to the FCC.
Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:
Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.
Harmonic frequencies to be measured:
Harmonics for 96.9 MHz fundamental
Harmonics for 97.9 MHz fundamental
Comments
193.8
195.8
290.7
293.7
387.6
391.6
484.5
489.5
581.4
587.4
678.3*
685.3*
US LTE Band 71
775.2*
783.2*
US LTE Band 5
872.1*
881.1*
US LTE Band 5
969.0
979.0
*Frequencies that fall within the mobile data LTE bands. Traces were recorded and saved for these frequencies.
All of that information, once compiled is attached to the FCC form 350-FM, which, once filed grants Program Test Authority.
BW TXT-600 V2 translator transmitters under test and measurement
