Groundplane vs. Dipole for LowBand

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mancow

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I realize they should be pretty much the same but are there any opinions on which would be better a groundplane or dipole?

I am thinking of making one similar to the one in this thread.
http://forums.radioreference.com/scanner-receiver-antennas/118428-lowband-homebrew-sputnik-groundplane.html

The plan is to hang it from the vertical element about 30 feet up from a tree branch. I plan on feeding it with RG-6 since it's a receive only antenna and I have a large spool the TV guy left behind.

I am interested in general lowband reception in the 32 meg range from any direction and was wondering if that would offer a more omnidirectional advantage over a dipole or is it worth the effort?
 

davenlr

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A dipole with have about 2.5db gain over a ground plane if a proper balun is used at the feed point (dipole is 75 ohm balanced, ground plane is 50 ohm unbalanced, coax is either 50 or 75 ohm unbalanced, so for a dipole you need a 1:1 balun to convert the balanced feedpoint to the unbalanced coax)
 

prcguy

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It can be the other way around. A 1/4 wave monopole over infinite conducting ground plane has a gain of 5.16dBi (Kraus) and a half wave dipole in free space has a gain of 2.14dBi (again, according to Kraus). In actual operation, a ground plane with 3 or 4 radials will have less than the infinite ground plane and at some point a dipole is a little better.

You must also consider height above ground and dipole orientation, etc, which can pull one antenna ahead of the other in gain.
prcguy

A dipole with have about 2.5db gain over a ground plane if a proper balun is used at the feed point (dipole is 75 ohm balanced, ground plane is 50 ohm unbalanced, coax is either 50 or 75 ohm unbalanced, so for a dipole you need a 1:1 balun to convert the balanced feedpoint to the unbalanced coax)
 

prcguy

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What ever happened to the green antenna with fold out legs? It should beat the pants off any dipole or ground plane when considering the entire band from 30 to 50MHz. At a spot frequency a dipole or GP should work slightly better.
prcguy

I realize they should be pretty much the same but are there any opinions on which would be better a groundplane or dipole?

I am thinking of making one similar to the one in this thread.
http://forums.radioreference.com/scanner-receiver-antennas/118428-lowband-homebrew-sputnik-groundplane.html

The plan is to hang it from the vertical element about 30 feet up from a tree branch. I plan on feeding it with RG-6 since it's a receive only antenna and I have a large spool the TV guy left behind.

I am interested in general lowband reception in the 32 meg range from any direction and was wondering if that would offer a more omnidirectional advantage over a dipole or is it worth the effort?
 

mancow

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The com 201b? That thing is giving me fits. I get nothing but broad band noise. I ended up taking it down off the roof today.
 

prcguy

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That sounds very strange, it should work like gangbusters. I compared one just last week to the military Shakespeare 30-512MHz vehicle antenna on a ground plane/tripod adapter and on 6m the COM201B measured 4dB better than the Shakespeare. That's measuring both antennas in the exact same spot with a spectrum analyzer receiving local 6m repeaters. 4dB is not a trivial amount.
prcguy

The com 201b? That thing is giving me fits. I get nothing but broad band noise. I ended up taking it down off the roof today.
 

mancow

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I think something must be wrong. I get nothing but broad streaks on the sdr and all legit signals are far below the gap vertical that is ground mounted.

As you know I was missing the top portion but I built my own. I can't imagine that is the problem but maybe.
 

mancow

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I picked up some 8 foot 1/2" aluminum angle, an outlet cover and some stainless hardware today. The driven element is just a piece of solid copper house wire looped around a bolt that threads into the antenna connector. I painted the whole thing with primer gray paint and hung it from a branch by the copper wire. It's fed with RG-6. It works extremely well.



 

prcguy

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Send it to me for testing, I'll give you my Fed-X account info when ready.
prcguy

I think something must be wrong. I get nothing but broad streaks on the sdr and all legit signals are far below the gap vertical that is ground mounted.

As you know I was missing the top portion but I built my own. I can't imagine that is the problem but maybe.
 

k9rzz

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Glad you got it running well. I was going to say, for lowband VHF - polarization is key for ground path stuff. A dipole will work just as well as long as it's vertical, just like your ground plane. Carry on. ~:^]
 

mancow

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What's the thought on radiator diameter? Would using a larger one such a piece of 2" pipe or some wire mesh rolled into a large tube increase bandwidth? Would it be worth the effort?
 

prcguy

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If you make the elements fat like 2" it will have more BW but noting drastic like covering 10Mhz. If you know how to drive EZNEC it will tell you if something like a 1ft diameter cage element made from chicken wire would give suitable BW over most of the band. If your going to suspend the thing anyway why not make the 30-90MHz bow tie shown in this article?

Radio communications, light infantry and MOUT

Its kind of similar to the OE-254 bicone I used to run on 10m and 6m plus VHF monitoring but I recently replaced it with a gain type 6m omni. The balun for the bow tie can be any 4:1 that covers up to 6m and in another article by the same guy he placed a 50pf or so cap across the 50 ohm side to provide a better match.

In the end the green antenna you have is supposed to be superior to the OE-254 and the bow tie and the author of the article even mentions that. Did you ever verify if its working ok?
prcguy

What's the thought on radiator diameter? Would using a larger one such a piece of 2" pipe or some wire mesh rolled into a large tube increase bandwidth? Would it be worth the effort?
 
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