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I've built a 2 meter ground plane antenna that works pretty well for RX and has a decent SWR on 2 meters with my Alinco. However, I'm experimenting with SDR quite a bit and would really like to build something for weak signals, especially CW and RTTY at less than 50 MHz.

To that end, I was thinking of building a large dipole, but I'm getting advice that says that an ungrounded end fed wire run up the side of the house would likely perform far better.

First, is this true? Second, if it is true, why is it true?

Note: I am planning to use this antenna for RX ONLY, so I'm not really concerned about frequency matching on the length of the wire or dipole arms.

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  • $\begingroup$ Alnico is a metal alloy. Did you mean Alinco (radio)? Just making sure. $\endgroup$ – Kevin Reid AG6YO Aug 12 '15 at 14:56
  • $\begingroup$ Whoops! Typo or autocorrect. $\endgroup$ – David Hoelzer Aug 12 '15 at 14:56
  • $\begingroup$ I added some bits to clarify that you're asking about HF antennas, even though you start out saying "2 meter ground plane". The frequency range is much more relevant than "for weak signals". $\endgroup$ – Kevin Reid AG6YO Aug 12 '15 at 14:59
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    $\begingroup$ I suppose I appreciate the "Hint" but if I had a hint I wouldn't be asking. What's the "other thing" you're talking about? My impression was that the current is induced by the incidental RF energy. $\endgroup$ – David Hoelzer Aug 12 '15 at 17:31
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    $\begingroup$ When you say "end fed", does that mean something other than a random length wire? Most any antenna will work for Rx. The thing to worry about might be the local noise pickup. A random wire will pick up all sorts of noise from stuff in your house. A horizontal dipole up in the air away from local power lines, etc. and fed with balanced line (twin lead) or coax should be best from the noise point of view. $\endgroup$ – Martin Ewing AA6E Aug 12 '15 at 22:20
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An end fed wire may not necessarily work BETTER for reception, but it should be effective and it is much simpler to put up. You might pick up a bit more hash noise from local RFI sources with an ungrounded configuration though, due to common mode currents induced on the outer shield of the feedline.

Feeding the wire through an isolation transformer, and grounding the end of the secondary winding opposite the antenna wire, and then also grounding the shield of the feedline at some distance from the antenna where the feedline enters your house would help reduce noise, and provide more safety for your equipment.

Other good configurations for reception of weak signals might be:

  • Run the wire close to the ground (6ft) with a counterpoise wire on the ground beneath, and then short the FAR end to the counterpoise wire through a large resistor and feed the two wires at the other end via a matching transformer and ground the shield of the feedline where it enters your house.
  • A fairly small loop of wire with a tuning capacitor at the point opposite the feedpoint, fed via a matching transformer. The loop can be rotated for best signal pickup or to null out local noise sources.
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Yes, a single piece of wire connected to the antenna socket of the receiver can be an effective receive antenna for many frequencies. A so-called random wire antenna doesn't need to be any more complicated than that.

Note that the spatial pattern of best reception will vary with frequency as the length of the antenna in terms of the wavelength changes. If there are particular frequencies or directions where you would like to optimize your reception, then you may want to consider a more sophisticated antenna such as a doublet or dipole

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