FM Broadband antennas are a compromise because they generally have less gain than tuned antennas, are more complicated, and take up more space. However, this antenna has none of those issues. The gain and radiation pattern appears to be almost the same as a tuned three-bay FM antenna.
We are finishing up an antenna project in Pittsfield, MA, this week.
Proposed W277CJ 60 dBu contourRemnants of Shively 6812 4-bay antenna
The project involved replacing a Shively 6812 tuned to 95.9 MHz (WBEC-FM) with the TFC2K so that the W277CJ 103.3 MHz (WUPE) translator located on the roof of the 14-story Holiday Inn on West Street could be moved to the studio location. In this case, having the translator in-house will save significant rent. The new antenna will continue to serve as a backup facility for WBEC-FM when the main site is off the air for whatever reason.
Single bay, Bext TFC2K antenna
The input power per bay is based on the antenna’s input connector. In this case, each bay has a 7-16 DIN connector and the power divider is a 7/8 inch EIA flange. Thus the maximum input power for this setup is 5.5 KW. The licensed output for both facilities is far below that.
3 Bays leg mounted on the tower
According to the manual, this antenna should be spaced at 0.85 wavelength, which is frequency-dependent. I chose a frequency halfway between the two (103.3 – 95.9)/2+95.9 = 99.5 MHz. The formula from the Bext general antenna manual is:
D = (300/F) x 0.85
Where D = the distance between center of radiating elements (booms) F - Frequency in Mhz.
Thus, D = (300/99.5) x 0.85 = 2.56 meters (or 8′ 5″)
As this is a series excited AM tower, some type of broadband isolation coil is needed to cross the base insulator. This one is simply a large coil of 7/8 inch coax, likely with a capacitor across the outer conductor to create a resonant LC network.
To me, it looks like a water heater. Since the ground is frozen solid, we made a temporary stand. We will have to come back in the spring to create a permanent stand or perhaps a unistrut mount to the wall of the ATU building.
Kintronic ISO-88P-78EIA-4C
In the rack room, the transmitters are combined into a Bext FDCSDC2 star point combiner.
Antenna combiner
Broadband sweep shows a good match across the entire FM band. I will be interested to see how it performs with respect to the Shively single bay 6812 on the roof of the hotel (103.3 W277CJ).
Return loss, Bext TFC2K 3 bay FM antenna
The return loss looks good on both 95.9 and 103.3 MHz. The interference noted in the sweep is from local FM stations including the main transmitter for 95.9 MHz.
We just finished our 3rd annual February ice storm. It is becoming somewhat of a tradition in these parts. After shoveling the driveway this morning, I sat down to enjoy my nice hot coffee. While doing that, I figured I would check some of the transmitters to see how things were going. That is when I noticed this:
The reflected power is much higher than normal indicating potential issues with the antenna deicers. I knew something was wrong after a quick call to the Burk, which stated the deicers were on. A quick double-check showed that the reflected power had increased by another 75 watts, so a nice drive to the transmitter site was in order.
Road to the transmitter site
Indeed, the controller had turned on the antenna deicers.
Antenna deicer controller
Using a clamp on amp meter, I saw almost no current on either leg of the 240-volt circuit. In the meantime, the backup antenna had 2 amps on each leg, which is normal. Then I noticed this:
Antenna deicer relays
The relay on the right shows signs of overheating.
I moved the Main Antenna circuit over to the aux antenna relay to get things going again. The current on each leg of the main circuit is 4.2 amps. Over the next 45 minutes, the reflected power returned to normal.
Other transmitter sites to the north in Albany have had similar issues. Unfortunately, those antennas do not have heaters or radomes. Thus, the only remedy is reducing power until the transmitter stays on.
I also noticed that when there is an antenna problem, the station does not sound as good as it normally does because of the bandwidth restrictions adding distortion in the frequency domain.
Another liquid-cooled GatesAir transmitter installation was completed for WYPR, Baltimore, MD. This unit replaced a Continental 816 which had a long life.
The area around the transmitter site is not the best neighborhood. The building was formerly the WBAL-TV transmitter site and was built circa 1947.
WYPR transmitter siteOriginal four-legged Blaw Knox self-supporting tower from the WBAL-TV daysPump station and HTF tubingWall-mounted heat exchangerFLX-20 transmitter
The transmitter room is a little tight, so it was difficult to get a good shot of the front of the unit.
The station is running HD Radio with -10 dBc.
This site has a strange 3-phase delta AC power configuration. The middle leg is at ground potential, and the other legs all measure 240 VAC to ground. I’ve never encountered that before. This is known as a corner grounded Delta, which gets rid of the high leg associated with most closed 3-phase delta systems.
Ultimately, all the leg-to-leg measurements are 240 volts, so the power supplies are satisfied. With these transmitters, the phase rotation does not matter because there are no actual 3-phase loads in the transmitter.
Inventory control, Home Depot Reisterstown Road, Baltimore, MD
I purchased this GPS Disciplined Oscillator a few weeks ago. The reason being, I wanted to make sure that this frequency counter was accurate.
Hewlett Packard 5315A Universal Counter
This Hewlett Packard 5315A was last calibrated in 1990. That made me suspicious. While I could send it back to Agilent and have it recalibrated, I thought it might be interesting to check it with a known standard.
When I connected the frequency counter to the 10 MHz GPSDO, it was -2.1 Hz off. At first, I thought perhaps the GPSDO was off; however, the spec for the LBE-1420 is 1 x 10-12 with a resolution of 1 Micro Hz. I let the HP unit warm up for 3 hours thinking maybe it was cold and would come back in tolerance. Nope, the frequency stayed about 2 Hz low.
It took about five seconds to find the full service manual online, which gives the alignment and calibration procedure in detail.
The first step is to use a DVM and check the +3, +5, and -5.2 power supplies. If they are off then adjust each accordingly. The next step is to check the +5 VDC pin on the Option 4 OCXO module and adjust as needed.
‘scope lead connected to reference oscillator pin
The calibration procedure for the HP 5315A is to connect a known 10 MHz reference to one channel of an oscilloscope and the output of the frequency counter OCXO to the other channel and look for slippage of the two signals. If the counter is on frequency, there should be no movement between the two waveforms. This is more accurate than trying to adjust the counter while looking at the frequency display on the counter.
Frequency alignment HP 5315A, Yellow squarewave trace LBE-1420 GPSDO, Violet sinewave trace 5315A reference oscillator
When I first connected it, the HP’s waveform was running backward at a pretty good clip. I adjusted the OCXO until there was no movement relative to the two waveforms. I let it sit like this for about three more hours before buttoning the HP unit back up. I am confident that the frequency counter is accurate +/- 0.3 Hz, which is good enough for my purposes.
LBE-1420 GUI
What I like about this Leo Bodnar GPSDO is that you can change the output frequency to any value between 1 Hz and 1.4 GHz. The output level is +13 dBm (per data sheet) with low phase noise, making it an excellent portable signal generator. The output is a squarewave, however, installing an LC type bandpass filter such as a Mini-Circuits SBP 10.7+ will round that out into a nice sinewave.
The Leo Bodnar website has a portable Windows executable for download, which can be used to program the output frequency and monitor performance.
I measured the output with my precision power meter; at 10 MHz it was +10.35 dBm. The low power output setting is about +5 dBm.
WWV carrier measurement with LBE-1420 as external 10 MHz reference
Another use for the LBE-1420 is as an external 10 MHz reference for test equipment. My Network Analyzer (and many other pieces of test gear) has an external 10 MHz input and if I use the spectrum analyzer to measure carrier or pilot frequency, it is nice to know that the test equipment is exactly on. I confirmed this by measuring the WWV carrier with my Siglent SVA-1032X spectrum analyzer using a long wire antenna.
Inexpensive Chinese GPSDO
Continuing with this interesting topic, I purchased a fairly cheap version from Ebay for further research. This particular unit is a clone of a BG7TBL, which is itself a clone. The interesting thing about these units is that they are using recycled OCXOs, which appear to be from decommissioned telecom equipment.
BG7TBL GPSDO block diagram
This diagram shows how these units work. The GPS signal is received by the GPS module, in this case, a uBlox M-7. The NMEA sentences and 1PPS are fed into the CPLD (Complex Programmable Logic Device). The NMEA sentences are also available on the RS-232 DB-9 connector.
GPSDO component side, GPS module lower left
The CPLD takes the output of the OCXO, in this case, a CTI OC`12SC38A, and compares the 1PPS from the GPS module to the 10 MHz from the OCXO module and adjusts the OCXO module by varying the voltage on the frequency adjust pin to keep it on 10 MHZ. It then sends the corrected 10 MHz and 1PPS signal out to BNC jacks. I found the 10 Mhz output level was +13.58 dBm as measured with my precison power meter. There is a built in bandpass filter, so the output is a good looking sinewave.
Judging by the CTI model number, it was made before 2015. There should be a date code on the bottom of the unit, but I did not feel like unsoldering it.
GPSDO OCXO
The one issue with this; OCXOs frequency drifts over time and eventually it will be out of the adjustment range. A closer look at the circuit board shows that it will accept several different OCXO modules. These modules run about $40-60 US new and $10-15 US used.
If an OCXO is suspected of being out of adjustment, they can be measured using the osciliscope method noted above.
