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Re: Topband: Using shielded CAT5 data cable as feedline for active anten

To: topband reflector <Topband@contesting.com>
Subject: Re: Topband: Using shielded CAT5 data cable as feedline for active antenna; benefits of multi-turn K9AY loop/SAL/etc?
From: Pete Smith N4ZR <n4zr@contesting.com>
Date: Sun, 3 Jan 2016 12:01:18 -0500
List-post: <topband@contesting.com">mailto:topband@contesting.com>
I should have been clearer - all his work is with small loops, typically 1 meter diameter. My interest was in the possibility of improving the performance of my omnidirectional active Skimmer antenna, currently a Clifton Labs 8-ft vertical. Since some of his antennas favor horizontal polarization, according to his writeups, my thought was that perhaps combining vertical and horizontal polarization might yield a net improvement in SNR.

73, Pete N4ZR
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On 1/2/2016 11:19 PM, Tom W8JI wrote:
2. He cites experimental data showing that coplanar crossed loops and multi-turn quad loops both offer very significant improvement in the recovered signalcompared with a single loop. See <http://www.lz1aq.signacor.com/docs/experimental-comparison-v10.pdf> to check whether I got this right. Anyway, it occurred to me to ask if anyone has ever tried multiturn K9AY, SAL or flag/pennant receiving antennas, and did you see something similar?

Be careful in what you might think the data means. The measurements are for an unmatched system, and apply to broadband untuned loops with "low impedance" loads.

In a case like that, the parallel wires reduce the impedance primarily by reducing reactance. It is no different than a thicker conductor, which would reduce reactance and increase current in the simple circuit.

This does not necessarily mean the loop would have a higher SNR, that would depend on how the amplifier "likes" the lower impedance and if external noise is limiting the system.

It does not mean more directivity. An even larger improvement in sensitivity would come from cancelling reactance.

If a small terminated loop had increased conductor size it would have more sensitivity, which means increased signal and noise pickup, because the termination and source resistances would decrease.

You can see this effect by modeling an EWE antenna, or any small loop. As the conductor is made thicker the optimum termination resistance decreases. This increases sensitivity, because radiation resistance remains the same and loss resistances decrease. You can pick up a few dB in sensitivity in certain cases.

If the amplifier or receive system is affecting S/N in a significant manner, it would improve S/N. If external noise is the significant factor in sensitivity, then it would pretty much do nothing.

This effect occurs in all sorts of lossy antennas. For example, if you paralleled two close-spaced Beverages (making them act like a single very wide conductor) sensitivity increases. This does not mean S/N ratio increases, because signal and noise from the antenna would increase at the same rate. It just means the level of signal and noise from the antenna is a bit higher.

If receive amplifier or system internal noise is helping set noise floor, then it would help S/N.

73 Tom


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