It’s a nice antenna, but DOES IT WORK? When you have the antenna all set to go, but your radio and duplexer are still weeks or months away from being ready, you can’t resist the urge to bring in the radio from your shack and give it a try. I brought my trusty Kenwood TS-2000 and power supply up to the transmitter site in early February to do just that.
But wait! This antenna is sitting about 100 feet away from a UHF antenna that is pumping out around 600 or 700 KILOwatts ERP of RF! Ouch! Once again, being a broadcast engineer is a handy thing, because I have a high-precision power meter available. The Agilent E4418B revealed that about 2.75 dBm of signal (1.88 mW) was being coupled into our VHF antenna, and back down the feedline. This was much less than I expected, and is probably largely due to the low band VHF antenna not being very effective as a pickup for the UHF energy.
Once the repeater’s duplexer is built and installed, there should be plenty of rejection of the UHF signals, so I’m sure it won’t be any trouble at all for the GE Mastr II’s tight front end. However, for my beloved TS-2000, I wanted to take every precaution, so I set up a diplexer and dummy load as shown. With the antenna connected to the common port, RF energy below 150 MHz is coupled to the radio, and the UHF energy goes to the dummy load. This setup dropped the power level from 2.75 dBm to -4.10 dBm (about a third of a milliwatt), and gave me some extra piece of mind that I wasn’t going to fry the front end of the TS-2000. Why still so much signal? There is 100 KW of FM broadcast at 94.5 MHz, several hundred feet below, and another strong FM at 107.9 only .6 miles away. The diplexer doesn’t give me much attenuation at those frequencies.
Batwing antennas are supposed to be very broadband. This antenna was designed for 60-66 MHz, and we’re going to use it at 53.67 MHz. That should be close enough to avoid major problems. A channel 2 (54-60 MHz) antenna would have been even better, but we’ll be grateful for what we have. My rig has an SWR meter, but it seemed appropriate to use a meter with more precision, such as a trusty Bird 43. I bought a new pickup element on eBay that was suitable for 100 watts on 6 meters, and fired up the rig. In the picture to the left (click to enlarge), the rig is transmitting on our proposed operating frequency of 53.67 MHz, and showing 96 watts output on the Bird. If you’re new to Bird directional wattmeters, note the direction the arrow is facing on the round pickup element below the meter face. You simply rotate the element to measure power traveling in the opposite direction. The next picture, to the right, shows the element pointing to the left, meaning that it is measuring the reflected power. The rig is again transmitting at the 100 watt setting, and there is virtually no reflected power! Also, if you click on the image to enlarge it, you can see that the rig’s built-in SWR meter shows just one “dot,” confirming that virtually all the power is being transferred to the load. Hopefully this means that the power is actually being radiated (an old ham friend of mine is fond of reminding me that a dummy load at the end of a feedline will show a low SWR, but it won’t radiate much!)
On the way up to the repeater site with the TS-2000, I had been chatting with one of the local hams on 2m about my plans to test the broadcast antenna with a 6m signal. It turns out that several other guys were listening also, so the instant I keyed the mic on 6m and identified myself, they all chimed right in! Everyone in the local Syracuse area reported DFQ (full scale, Dead Full Quieting) no matter how low I adjusted the power! A few guys farther out could hear the difference when I dropped to 5 watts, but it was still strong for most of them.
Going the other way, I was receiving very strong signals from just about everyone. The more distant stations were just a little lower on my meter, but still pretty strong. The noisiest signal I heard that day was from a tiny Icom T90 HT, 25 miles to the west over hilly terrain. Seventy miles to the north over flat terrain, a station with an outdoor antenna reported S9+ no matter what I did.
It was a short test, not particularly organized, but the good results generated a LOT of enthusiasm for the 6m repeater project. A number of hams that I know to have 6m rigs were at work during the afternoon test, so they couldn’t participate. I decided that a better-publicized test would be a good idea, probably in the evening. Hopefully you will read this post in time to join us! My next post will give the details.