modulator

[Electronic M]

I'd go with a UHF bowtie dipole. Those loops are shorts. 75ohm cable is transmission line. if you short transmission line and it is at the wrong fraction (or multiple of) the wave length of the transmitted signal then some or ALL of the transmitter signal power will not be transmitted but instead reflected back to the transmitter (which can damage some transmitters).

Also I'm leery of using matching transformers/baluns designed for receiving antennas...The transmitter power may be sufficient to damage them, and cause poor or NO performance.

I'm VHF exclusive right now, and this video has been my guide.
https://www.youtube.com/watch?v=z4dhRt7tmd4
I'd give it a try on VHF this way first, then start trying UHF if I were you....That video you posted is the first talk of using a BT mod for UHF that I've seen.

[NoPegs]

Originally Posted by Electronic M

I'd go with a UHF bowtie dipole.

I'll also the praises for good old oven-rack in fancy-dress with bowtie(s) style of antenna. Damned good at what they do.


The discussion on baluns in this thread so far is spot on. That is to say, your bog-standard 75 to 300 ohm TV balun is absolute crap if you're trying to transmit power through it. (TBFH, the average TV balun is just crap, full stop.)


http://www.kyes.com/antenna/balun.html has a good writeup/table on the kind of losses you'll be facing. -3dB means you're losing HALF your power, -6 means you're losing THREE-QUARTERS. So for VHF use, you'll be lucky to see 40% of what you put into it come out the other end. For UHF it's a roulette wheel style thing where on a given channel you might only lose 30% of your power, but if you slightly alter the length of the cable at some point suddenly you're losing 90% but on a previously unusable channel you're getting great throughput.

The page I linked also includes information on rolling your own 4:1 balun, but this link will make things a bit easier. http://n-lemma.com/calcs/dipole/balun.htm The important thing here is that you do need to know with some level of confidence what the velocity factor of the cable you're using actually is. Dig through your coax stash until you find something that has a series number on it that you can turn into a spec-sheet with google. (If you see Belden, you've got it made.) Once you plug the known velocity factor into the calc, next you'll plug in the video carrier center frequency(Not the channel center frequency.) for your channel of choice, and the calculator will return a length value with a hilarious number of digits east of the decimal point. For your purposes here you can round to the nearest 0.1" and be OK. Good cable-prep and soldering skills will help, but if this is your first time making RF signal transmission line connections, spending half an hour practicing on a few feet of junk coax is a good idea, so that you get a feel for where and how to begin the cutting/stripping process so that your finished length is as close as possible to the ideal desired length.


I'm also in agreement that you probably want to attenuate the output of the modulator if you're direct wiring it to a set.