Sunday, March 5, 2017

Non-resonant versus resonant antenna for receive only use

Well... that's not what I expected

I've been playing around with my SDRPlay and have been thrilled with its sensitivity when connected to my attic antenna.  My attic antenna is a 68 foot doublet that weaves around my attic and is fed with 450 ohm ladder line.  I use a 1:1 balun connected to the ladder line and a short run of coax with the RF-front-end-protection  to feed my SDR.  

My Doublet antenna is not resonant on any ham band and requires matching to bring the impedance in line with something my transceiver wants to transmit into.

My outdoor antenna is an 80m OCF Dipole (aka Windom) in the 1/3 configuration, meaning that the short side of the dipole is 1/3 the length of the long side.  Its been a good performer for me and is resonant on 80m, 40m, 20m, 17m, 12m and 10m .  While using the indoor Doublet antenna as a receive antenna for the SDR (with front-end protection and a relay to switch out the SDR during transmit) I use the Windom connected to my transceiver.

This configuration has worked well and the SDR feeding HDSDR as a panadapter has been a real boon to finding stations quickly as well as being a wideband audio feed to CWSkimmer.

HDSDR fed by my SDRPlay

For grins I wondered how much better the Windom would be as a receive antenna for the SDR on the resonant Ham bands... Well it was between 6dB to 10dB down on reception on most bands when used with my SDR.  I tested this by alternately connecting the Windom and the Doublet to the SDR for repeated tests.

This doesn't make sense to me.  I assumed that a resonant antenna (and one that's much longer) would be a better antenna than the Doublet for the SDR but it's not working that way in my case.  

When I use the Doublet with the transceiver it is used with an autotuner.  In general my A/B tests using my transceiver between the Doublet and the Windom are about 3dB to 6dB weaker for the Doublet on receive but I often get better RBN signal reports using the Doublet by a few dB on transmit.

I'm really perplexed.  I expected the Windom to be a better receive antenna for the SDR.

I have a lot more to learn about antennas.

That's all for now...

So lower your power and raise your expectations

Richard, AA4OO

1 comment:

  1. That's curious indeed, Rich. I'm no antenna guru but normally an indoor antenna is a poorer performer. I wonder if the tuner is introducing the loss since you aren't using it with the SDR. The tuner only protects the radio during transmission, it doesn't make the antenna perform any better. Perhaps there's some loss there. Just a thought. Cheers de Scott ve3vvf