Impedance matching - RSP1A

Useful information regarding antennas for SDR products.
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Tom Atkinson
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Impedance matching - RSP1A

Post by Tom Atkinson » Sat Jan 19, 2019 3:44 am

What is everybody here doing for impedance matching between antenna and RSP1A? My antenna is nominally 300 ohms and I am using trial and error to find the optimal transformer. Feedline is 300 ohm twin lead. Antenna is a "horizontal loop" (actually in triangle shape) of 75 metre perimeter. Modelling by Phil (VK7JJ) shows these antennas are nominally 300 ohms for most of the SW spectrum.

Am making these transformers out of ferrite toroids and experimenting with the number of turns. Started with 11/27 turns (based on internet research) but have found that, with the particular ferrite material I have, 5/12 turns is giving a better signal to noise ratio.

Also, am wondering whether it would be worth making a 1:1 transformer, to be placed at the antenna for the further elimination of common-mode noise. Worth it?

Interested only in the SW spectrum.

Last edited by Tom Atkinson on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Re: Impedance matching - RSP1A

Post by g1hbe » Sat Jan 19, 2019 9:18 am

I think hands-on trial-and-error testing is probably best if you don't know the characteristics of the ferrite. A 5/12 (2.5) turns ratio will give an impedance ratio of 6.25 which ends up with 48 ohms at the RSP, assuming you have the transformer the right way around. So that's bang on, considering the antenna impedance will vary over the frequency range and so will the RSP.
Don't get hung up on achieving a perfect match, it really makes little difference as long as the mismatch isn't massive.

BTW, ferrite rods as used in radios are made down to a price and I've found they don't all work well below about 200kHz or above a few MHz. Proper ferrites are recommended - I use binocular cores of mix 43 for general use and mix 73 for low frequencies.

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Re: Impedance matching - RSP1A

Post by ON5HB » Mon Jan 21, 2019 5:42 pm

It's my experience that most receivers are not that sensitive to antenna impedance.
As such I do not care what my antenna does as long as it's receiving, you are mostly reducing signal levels anyway to avoid overloading. :lol:

Also, in most cases the SDR radios get too much signal anyway to be a problem is standing-wave is off, never noticed problems with it.

That being said, a common-mode-chocke is needed in almost all cases to keep QRM getting up the coax and into the antenna-feeding-point.
You do not need a 1:1 balun for that, just a plastic bottle of about 10-15cm and wind coax around it 10-15 times and you are done.
Other way is putting ferrite on the coax near the antenna-feeding-point but beware, you may need a lot of them, mine are about 3 meters long, but then I transmit on it as well with 1KW, then there is never enough ferrite :D

Cheap an effective way is a bottle.

Also called poor-man's-balun or ugly-balun :lol:

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