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 80 mtr full size ground plane 
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Post 80 mtr full size ground plane
Hi again Steve,
Full size 80 metre GP fed with RG58 co-ax Got a new toy (rig expert aa-30)? and measured the following values
Measured my 80 mtr ground plane a number of times and it came up with the following figures

at 3.650 Mhz...........swr = 1.8 or 1.9 (it's windy)
Z =76
R = 69.1 Ohms
X=33 Ohms
C = 1360 pfs
Any ideas of how to improve it?
John G4PKP


Fri Sep 14, 2012 4:23 pm
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 80 mtr full size ground plane 
Silent Key

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Post Re: 80 mtr full size ground plane
John,

The resistance looks quite high for a full-size 80m quarter-wave vertical. Assuming it's ground mounted, you need more copper in the ground - more radials!

73,
Steve G3TXQ


Fri Sep 14, 2012 5:37 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
Feed point is about 10 feet off the ground Steve. It's tied to a halyard on a 80ft. tower with 4 sloping 1/4 wave radials.
John


Fri Sep 14, 2012 6:51 pm
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 80 mtr full size ground plane 
Silent Key

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Post Re: 80 mtr full size ground plane
John,

Was that impedance measured right at the feedpoint, or through a length of coax? If the latter, what length of coax?

Steve G3TXQ

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Fri Sep 14, 2012 7:28 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
OK Steve, all bets are off. Unknown length of co-ax, it was long enough to reach the shack! I'll measure at the feedpoint tomorrow and take it up again with you. Tnxs for your interest.
John


Fri Sep 14, 2012 8:22 pm
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 80 mtr full size ground plane 
Silent Key

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Post Re: 80 mtr full size ground plane
John,

The reason for asking was that the length of coax will transform the impedance; so what you measured isn't necessarily the feedpoint impedance. You can't really deduce anything about ground losses etc without knowing the coax length, or making a measurement right at the feedpoint.

73,
Steve G3TXQ


Fri Sep 14, 2012 8:40 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
Steve, Halelujah (or summat like that!) Figures at the feedpoint, feeder disconnected.
SWR @3.670 =1.7
Z= 30 ohms
R=29.6
X=4.9
L= 213 Mh

Does it make sense now and if so, what now?
John


Sat Sep 15, 2012 3:50 pm
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 80 mtr full size ground plane 
Silent Key

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Post Re: 80 mtr full size ground plane
John,

That looks pretty close to what you should expect for a quarter-wave vertical.

What do you mean "what now"? What don't you like about the results?

Steve G3TXQ


Sat Sep 15, 2012 4:24 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
Steve, Just wondered if it needing shortening slightly, is it too long?
John


Sat Sep 15, 2012 4:28 pm
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 80 mtr full size ground plane 
Silent Key

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Post Re: 80 mtr full size ground plane
It's very close to resonance at 3.670MHz. The residual reactance of 4.9 Ohms is pretty small - equivalent to an offset in tuning of about 20kHz; it's really not worth worrying about.

73,
Steve G3TXQ


Sat Sep 15, 2012 4:36 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
Where would you prefer having the lowest reading, higher in the band nearer the DX window?.... any listening I've done that low down is usually UK/European.

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Sat Sep 15, 2012 4:49 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
Yep, I would have liked it a bit higher cos I only use one frequency, 3.789 but as Steve intimated, if it's not broke, don't fix it. I think I'll leave well alone and let the ATU take care of any slight mismatches. I wonder if its worth having any more sloping radials?
John


Sat Sep 15, 2012 4:55 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
It's a fair bit away so you may have to trim some off it, have you checked what the SWR is next to the DX window?
Regarding elevated radials I read an article some guy did were he found no marked improvement using more than 4, if anything he found performance started to degrade the more he added, I've since discarded the art6icle but remember it was an American amateur and for that reason have only 4 elevated radials on my own 40m vertical.

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Sat Sep 15, 2012 5:03 pm
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 80 mtr full size ground plane 
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Post Re: 80 mtr full size ground plane
Measured SWR as follows 3.550= 2:1.......3.670 :1.7...........3.740......2:1 then upwards!
John


Sat Sep 15, 2012 5:35 pm
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 80 mtr full size ground plane 
Silent Key

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Post Re: 80 mtr full size ground plane
John,

If you want to shift the resonance, it will need about 15" taking off.

Like Gerry said, there's no significant benefit in going beyond 4 elevated radials. The definitive work was done by N6LF and written up in several QEX articles:
http://www.antennasbyn6lf.com/

73,
Steve G3TXQ


Sat Sep 15, 2012 6:11 pm
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