Agile modulator antenna Q's

[CJVx]

I have a BT AM-60 modulator set to channel 7. Currently I have a coax connected to a 75ohm-300 ohm matching transformer, then connected a pair of rabbit ears on the 300 ohm screw terminals. Both telescoping antennas set to measure 15.8" each.


An alternative would be to connect this dipole pair directly to the coax center and outer (gnd). not sure if this would be correct or be beneficial to the above setup. Comments?

Also, I want better onmidirectional performance, so if I were to make a loop, how long of wire would i make the loop out of for channel 7, and would I:
A: connect the loop to center of coax to outside of coax (GND)
B: connect coax to matching transformer, connect loop to 300 ohm terminals of matching transformer.

Which way is correct?

[Electronic M]

On my dipoles and the one shango66 made in his excellent video on the subject there's no matching transformer and the dipole elements are connected directly to canter and shield (I opted to connect to a female connector so I can change the coax feed line length).

Note most dipole length formulas are written for 50ohm feedline since that is most common in Ham radio and communications applications. In his video shango66 shortens his antenna to match it to the 75ohm coax... you can tune any 50ohm dipole to 75ohm by reducing the length by a fairly constant percentage...if you look up his video and calculate what percentage of the length he trimmed you and apply that percentage to your antenna to match impedance.

Technically it is a good idea to have a 1:1 impedance ratio balun if you want to feed the dipole directly from the coax, but you can often get away without one like I and Shango66 have done. Your 300ohm to 75 ohm 4:1 matching transformer is technically a balun, but it is somewhat lossy by design and you'd have to track down a 300ohm dipole equation to make your dipole match.

Someone else will have to chime in on loops since I haven't built one for a remotely similar application.

[AlanInSitges]

Sorry for sort-of hijacking this thread but figured it was better than starting a new one.

I'm going to be using UHF channel 25. Long story, can't really change it, that's what I've got to work with. I'm getting a usable signal with just a short piece of wire to the center conductor of the modulator's output, but of course want to increase my range and quality as much as possible.

I've been reading up on all the options for a transmitting antenna. It seems like I can either:

1. Make a dipole and connect coax to it, with element lengths of about six inches (it's 505MHz).

2. Use a UHF hoop antenna either connected to the coax directly or with a 75-300 balun in between.

If I want maximum gain it seems like I could experiment with building a yagi or a reflective array, but I'm not clear if all that I'm reading applies equally to transmission - most of what's online is about reception. Is it that simple? Make a reflective array with say four bowties and a reflector and then point it, say "down" from the top floor of my house?

[Electronic M]

You want to stick with a fairly omnidirectional design. That gain comes at the cost of a narrow signal path.

If you are playing with the same power as a BT AM60 modulator you can cover half a block radius easy with a good omnidirectional ant.

Unless you have a SWR meter or some other means of measuring match at frequency I'd be wary of connecting a UHF loop directly.