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Author Topic: SWL Random wire question  (Read 2138 times)
2E1CLS
Member

Posts: 23




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« on: December 21, 2012, 04:37:21 AM »

Hi, I currently have a random wire up in an inverted L, It goes from a 9.1 unun up parallel with my house and across my garden to a small tree the approx length is 50ft, at the unun is a small ground system and the antenna is fed with 50 ohm coax into my house to help reduce noise!

I use this antenna to SWL on as much of the HF spectrum as possible, but I cant get any more length without bending the wire in a right angle to another small tree which would give me another 20ft.

My question is would I be better sticking with what I already have or would the extra 20ft despite being bent 90 degrees be a better antenna?

Thanks Carl M6CLS.
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K5LXP
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Posts: 4506


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« Reply #1 on: December 21, 2012, 06:00:00 AM »

Any antenna operated over a wide frequency range will exhibit a varying pattern and impedance over that range.  The 9:1 balun really doesn't fit this application, nor does a 50 ohm feedline due to mismatch at all but a few select frequencies.  20 extra feet will change the Z and pattern a bit at the higher frequencies but being "random" you're exchanging one arbitrary pattern for another.

Short answer to your question is 20 extra feet isn't going to make much of a difference in overall receive performance.  You could model it and see if the resultant change in pattern would remarkably affect your results in a particular direction on a specific band but for "general purpose" SWL a 50ft wire is going to work pretty well.

Another way to look at it is it's just wire.  Put an alligator clip on the end, add the extra length and try it both ways.


Mark K5LXP
Albuquerque, NM
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K7KBN
Member

Posts: 2813




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« Reply #2 on: December 21, 2012, 07:16:44 AM »

Receivers don't care.  As a teenaged SWL in the late 1950s, I ran a piece of #22 wire (orange with a blue tracer, actually... Grin) from one corner of my bedroom ceiling, held up with a thumbtack, passing near the ceiling light (another thumbtack), and finishing in the kittycorner, right above the SX-99 with another thumbtack.  I connected the end of this 15 feet of wire to the antenna terminal of the receiver.  I could hear the entire world with that.  How did I come up with ~15 feet?  That's what fit.

Granted, there were a lot of really high-powered SWBC stations back then, but the rule for back then still holds true: just a piece of wire will work well for receiving.  Don't worry about SWR (or Baluns) for receiving antennas.  They absolutely do not enter the picture.  Just don't try transmitting with such antennas without a bit of forethought.
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73
Pat K7KBN
CWO4 USNR Ret.
N8CMQ
Member

Posts: 373




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« Reply #3 on: December 22, 2012, 04:16:23 PM »

A tuned loop antenna might be a better choice.
You can have a small receiving loop with a switch
to change bands and a variable cap to fine tune it.
They can be small enough to fit inside your room,
plus they can be rotated to get maximum signal,
or minimize interference.
Some DX chasers use such loops for their receivers.
Antennas for receiving follow the same rules as
antennas for transmitting, but antennas for receiving
don't have to worry about high voltages or currents,
so the components can be smaller without worrying
about arcing and failure in use.
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