Using a TV antenna directly as an amateur radio transceiver antenna.
Many websites and magazine articles exist wherein the author has disassembled a TV antenna and re-used the parts to construct a yagi-style antenna. Based on the feeding arrangement for the elements, it appeared that the basic TV antenna design is that of a log periodic dipole array (LPDA), which has as a part of it’s design, a very broad bandwidth (lowest frequency dipole to highest frequency dipole), and some marginal gain (less than a yagi, but far more than a vertical). When an antenna (or several) became available, this seemed like an opportune time to test this theory, since this would be the only antenna I was going to get up before winter.
Once I got all the elements unfolded, I set the antenna in the driveway on top of a stack of boxes that were available from unpacking after moving. A length of 75 ohm coax went to a few appropriate RF adaptors, then to my MFJ-249B antennalyzer. Yes, there is a mismatch form 75 to 50 ohms. No, I don’t care to take that into account. At this point it SHOULD be noted that I had a standard TV type 75-300 ohm balun at the feedpoint to the antenna. While this will work fine for my test, there may be some challenges when transmitting – I wouldn’t recommend using that particular balun for more than a few watts without testing the power handling capabilities. Then again… they’re cheap enough that one could test it, monitor VSWR, and, if there are no adverse effects, just use it… but I digress.
Without making any real notes, I spun the MFJ frequency dial from about 30 MHz to 174 MHz. I started to get a bit excited when I saw a null at 54 MHz (channel 2). I saw a few other nulls as I increased frequency, but I also saw some SWR peaks between those nulls… peaks ranging from 5:1 to 10:1. So I took a look at the nulls I observed:
What did this correlate to? Well – it looks as if the antenna is NOT an LPDA, but rather a set of dipoles tuned to the individual TV channels, but FED like an LPDA (with the alternating elements, and crossover feed arrangement between elements). The frequencies I found correspond to individual VHF television frequencies. I saw a small null at 144 MHz, but that’s likely the 72 MHz element offering some small degree of matching. I only have to assume that frequencies above 174 MHz continue to work this way, since I don’t have a way to test them without using a transmitter and VSWR meter.
Well, I don’t want to tear this thing down to build a yagi… I don’t have a situation where I need the gain at this point. I do want something broadband, though. I could look into a small LPDA, but that’s a lot more expensive than making this thing work for me. As of now, I’m considering leaving the 52 MHz element alone – that’s close enough for 6 meters. I am also considering cutting the remaining elements for 146 MHz, 154 MHz, 222 MHz, and 440 MHz. My theory here is that most of those elements will act as a dipole for frequencies I want to transmit on. The 154 MHz element is just to provide a better antenna to connect my scanner to, since there is still some activity in the VHF high band region. I should have enough elements tuned for 470 MHz and above already to give some fair performance in the higher end of the UHF scanner-geared spectrum.
I have also considered stripping off some of the parasitic elements, just to “clean up” or “slick” the antenna a little, but that will come after I trim the elements….
Success. That’s really all there is to say! I began trimming, and one thing I DID notice was that, even though the 2 meter element (I left the longest element alone) was about perfect in length, the VSWR response did not budge form my original test. I had a feeling there was more to the physics of how these antennas operate than I knew, so I chopped the rest off per my table. As soon as I finished, I hooked the antennalyzer back up, and lo and behold… 144-148 MHz was all below 3.0:1, with a null at 146, 156 MHz had a null (missed that one by a bit), and 50-54 MHz was below 3.0:1. I don’t have a GOOD (easy) way to test UHF, but I’ll take the little victories.
Once I finished testing, I began to “slick” the antenna a bit, removing the fan elements – and looking hard at the other parasitic elements. After removing the fans, I picked the antenna up, and it balanced almost perfectly at the mounting point – something it did NOT do before, so I have made a definite improvement on more than one front!
While I no longer have the antenna, but it would certainly be interesting to sweep it with a VNA… I didn’t have the VNA when I did this…