Antenna recommendations are useless - or are they?
Normally antenna recommendations to match to a radio are pretty much pointless - like "best antenna for Icom R75?"
At the risk of turning this into an antenna thread, the Commradio deserves a mention about it here as it differs just a tad from a conventional analog radio.
We all know that everything from a coat-hanger to a 5-acre farm "works", along with every contentious piece of advice that goes along with all that. Much of this advice ignores the fact that you are an SWL who cannot transmit to activate remote tuners, or have a shack full of test-equipment.
Unless you know exactly what you are doing beforehand, I can recommend a "type" of antenna due to the way the Commradio sdr works. And that type is the "non-resonant" antenna matched typically by a transformer at the feedpoint with a reasonably short run of coax, like no more than 50 feet or so.
WHY?
Unlike a conventional analog receiver, you may not hear much of a difference in the noise floor when there are not many active signals on the band. You may not hear much of any noise-peaks or nulls when using a manual tuner, or trimming or cutting the antenna itself. Essentially, the signals are either there or they are not, so it may be hard to figure out if you are making progress especially if the bands are already weak to begin with.
Since an SWL does not have transmit capability, remote tuners are out of the question, and shack-tuners are hard to adjust by ear without hearing peaks and nulls. Of course, an antenna analyzer would do wonders. But do you know how to interpret it, as mere SWR readings are not really an indicator of efficiency?
The goal here is to provide relatively good multi-band response, without a lot of test gear. The non-resonant types provide that. Fortunately, as an SWL we don't have to worry about matching a transmitter output, so using a transformer at the feedpoint works reasonably well even with swr values that a transmitter might require it's own internal tuner or external tuner to match to allow for operation.
As swl's, we know that antenna reciprocity only goes so far - the biggest improvement for swl's comes from increasing the signal-to-noise ratio, not necessarily obtaining the purest match. Directivity, placing the antenna away from noise sources, killing common mode coax current with chokes may be more important to us than chasing a "perfect match".
So what is the typical non-resonant antenna? A 4:1 UNUN at the base, and a length of wire anywhere from say 8 feet to 120 feet, telescopic whips long or short, helically wound our linear etc. The key here is the transformer and a relatively short run of coax. The feedline should be rf-choked, unless you are specifically trying to make the coax do double-duty as a transmission line and a ground radial.
Examples - I've used my own hand-wound 4:1 unun on pvc attached to a line run up a non-conductive fishing pole. Later I tried an LDG 4:1 unun. Even later a balun-designs unun each with increasing efficiency. I use a commercial product now with an internal hybrid 5:1 unun at the base for the most part, but hesitate to mention it as that usually becomes a thread-destroyer attracting sharks, naysayers, and hard core dx'ers who don't take into account ALL the variables of the environment in use.
There are quite a few of these antennas designed for amateurs, yet their non-resonant nature makes them good for my utility listening as well, and of course great for portable use as I can configure them into whatever environment I take the Commradio. I get good general coverage without having to take a lot of test gear with me to "do it right".
Note - I'm having so much FUN with this little radio that I picked up a second one for auxiliary duty, despite my desire for some added features or tweaking. I guess that says it all for me.
