Log-periodic antenna for UHF

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bicyclerepairman
Posts: 4
Joined: Mon Jan 04, 2021 12:45 pm

Log-periodic antenna for UHF

Post by bicyclerepairman » Mon Jan 04, 2021 12:51 pm

I'm thinking about building a UHF log-periodic antenna for my SDR. I want something directional with a wide frequency response (let's say, 300MHz - 1GHz) and this seems to be about my only option. I'm a little stuck on how I should connect it to the RTL-SDR. It seems a balun is the ideal option, but are hard to find at UHF. I also can't do any of the wavelength-stub style systems, because of the wide range of operating frequencies.

Do I need a balun? Do they exist at the frequencies I'm interested in? What would happen if I just connected the antenna with a short line of coax to the RTL-SDR?

Any thoughts appreciated!

VE3EAR
Posts: 49
Joined: Sat Oct 05, 2019 10:13 pm

Re: Log-periodic antenna for UHF

Post by VE3EAR » Thu Jan 07, 2021 4:49 pm

I've had reasonably good results with a number of antennas that required a balanced feed, by using cable-TV grade 75 ohm to 300 ohm baluns. The ones I'm using are made by Magnavox and have a
blue coloured plastic cover over an aluminum enclosure. The 75 ohm side is an F female, while the
300 ohm side has short leads terminated with fork-style terminals.

73, Bruce, VE3EAR

bicyclerepairman
Posts: 4
Joined: Mon Jan 04, 2021 12:45 pm

Re: Log-periodic antenna for UHF

Post by bicyclerepairman » Sun Jan 10, 2021 11:14 pm

Thanks for the reply - that's really useful information thanks. I had another thought - is there an active solution to this - ie, some kind of wideband amplifier that takes a balanced input and has an unbalanced (coax) output?

bicyclerepairman
Posts: 4
Joined: Mon Jan 04, 2021 12:45 pm

Re: Log-periodic antenna for UHF

Post by bicyclerepairman » Thu Jan 14, 2021 10:15 am

I had another thought - does anyone know how UHF TV log-periodic antennas deal with the balanced feed from the antenna elements? Do they contain a balun, or do they just connect the coax to the pairs of elements?

snn47
Posts: 253
Joined: Tue Dec 27, 2016 11:00 pm

Re: Log-periodic antenna for UHF

Post by snn47 » Sat Feb 13, 2021 10:43 am

Depends on the type of feed line.
If you have a symetrical feed line like a chicken wire (two parallel wires spearated by some form of plastic), you don't a Balaun at the antenna but at the end when you convert to a coax system, this is why early TV had the Balaun inside the TV-receiver at the symetrical input connector.

If it has a coax feed line it is inside a box whre the coax connecttor is mounted, unless they were so cheap that they did not use it.

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