eBay and other sources list a range unpowered (not active) antennas for general-coverage use. These are electrically-short whips or loops that are ostensibly impedance-matched to 50-ohm coax and feed the 50-ohm antenna input of receivers. They boast flat response from kilohertz to maybe 30 megs. Does anyone here have actual experience with these, to the extent that you can judge the relative performance against long-wire or active antennas? Any info on what sort of matching network is used on these would be welcome too.
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The Radio Board.org
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I'll be a way for a few days but will definitely fire this up before or during the coming weekend. That translated blog is interesting, and brings up a point I'd wondered about; that is, the length of the lead-in. The antenna comes with what looks like 20-30 feet of RG58, and the instructions don't touch on additional length. That's probably enough to get out of a noisy home environment, but not enough for a roof mount here. I would think that, "matched" to a 50-ohm line or not, additional cable capacitance would tend to negate the advantage of any passive impedance-matching in the sealed antenna base mount. I ran across a rather comprehensive overview of active E- and H-field antennas at g8jnj.net that seems to answer a number of questions.
@inojim wrote:
I was intrigued to notice that this antenna was made by a well-known Japanese company, so I figured there must have been some reasonable engineering effort that went into it. I did a little hunting and found this Japanese blog that did an impedance measurement on the antenna. Its operation looks complex.
Google translate link:
https://blog-livedoor-jp.translate.goog/kerokeronyororo/archives/74987442.html?_x_tr_sch=http&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_pto=wapp
From the Google translation:
I agree. An active whip generally loads the short antenna with several megohms; I don't think you can drive 50 ohms very well without a FET follower. I do have one of those whips on order, however, and will be interested in trying it out. Not cheap.
http://www.apexradio.co.jp/303WA-2_e.html
I've had variable success with active whips in the past, the best being a McKay/Dymek back in the '80s. It's probably still on the roof of the house I was living in back then.
One question on electrically-short antennas, perhaps you know the answer. If one does use a very small whip, like just part of a circuit board as in the popular PA0RDT design, would a correspondingly-small electrostatic shield (maybe two-foot diameter wire screen) grounded and placed underneath the antenna, not as a ground plane but simply as a shield, attenuate noise from directly below the antenna without compromising performance? I'm thinking of noise radiating from house wiring in the attic, where the antenna is on the roof above. Ideas?
Sounds like a dummy load.
You could use a balun to go from the Hi-Z antenna to the Low - Z receiver, and still be not active, but I would think such a device would need active elements to be useful with a short antenna.
i use a balun to reduce the mismatch between my end fed wire here at the lake, and the 50 ohm input on my homebrew receiver, and it helps some. But my end fed wire is about fifty feet long and high in the air.
73,
Win W5JAG