[UK-CONTEST] 80m antennas

Thu Apr 22 07:28:25 PDT 2010

>>> The exact figure depends on ground conductivity

Ah-ha! I never thought of that. Yes, one would get a very different result comparing a dry desert -vs- a wet ground!

James got to grips with EzNEC but I stick with MMANA (unfortunately) so the ground modelling isn't too good in that department. 

what do you think the real-world, average-uk-ground impedance would be at say 20 feet for 80m?

(I'll send to the reflector since this is good stuff)

Callum McCormick
t: 07976 631881
http://www.m0mcx.co.uk/

-----Original Message-----
From: g3ory at lineone.net [mailto:g3ory at lineone.net] 
Sent: 22 April 2010 15:16
To: callum at mccormick.uk.com
Subject: Re:80m antennas

Callum,

Sorry to say that it does not work like that. It would, if the earth 
was a much better conductor than it actually is.  
You are correct in identifying that the mutual impedance between the 
antenna and the earth reduces the feed point resistance of the antenna. 
Since the radiation is proportional to current in the antenna and not 
to the power , there will be NVIS gain as a result. Unfortunately, the 
earth is an insufficiently good conductor to give the results you 
predict.  For a half wave dipole at resonance, the feed point 
resistance rarely drops below 50 ohms as the antenna is lowered towards 
the ground (recall it is 72 ohms in free space).  The exact figure 
depends on ground conductivity. Shorter antennas will exhibit lower 
resistances but don't have as much vertical gain in the first place. 

Try a bit of modelling to see what that tells you. Eznec does a 
reasonable job with modelling earth (a can of worms in itself) .  The 
differences in signal strength are not large and probably impossible to 
observe 'on the air' without having two carefully erected identical 
antennas (except for height) for comparison. Even then it would be 
pretty hard to tell the difference between an antenna at 50 feet from 
one at 75 feet.

If you still don't believe me, then check out Fig 6.17 in Les Moxon's 
'classic' book.

73 Bob
G3ORY

>----Original Message----
>From: callum at mccormick.uk.com
>Date: 22/04/2010 11:28 
>To: "UK Contesting"<uk-contest at contesting.com>
>Subj: Re: [UK-CONTEST] QRS and QRP
>
>>>> I would rather use a "bent" 133ft dipole at 10ft than a G5RV at 
40ft!
>
>Any horizontal antenna at 10 feet, certainly from half-size through 
to
>double-sized) will outperform a similar antenna at 40 feet on 80m for 
NVIS,
>close working. There will be negligible differences on gain for 
anything in
>between. Be aware that if you really did run your 10 foot antenna, 
the
>impedance would be around 4 ohms, hence most people aim for 25 foot 
plus -
>just to get the impedance up to above 25 ohms.
>
>I guess you knew this anyway.
>
>Callum McCormick
>t: 07976 631881
>http://www.m0mcx.co.uk/
>
>
>
>-----Original Message-----
>From: uk-contest-bounces at contesting.com
>[mailto:uk-contest-bounces at contesting.com] On Behalf Of Rob - G4LMW
>Sent: 21 April 2010 22:42
>To: 'UK Contesting'
>Subject: Re: [UK-CONTEST] QRS and QRP
>
>Tom
>
>Was your QRS station at the top end of the section (3560+-) - the 
designated
>
>"QRS Corral"?
>
>Like Danny, I always slow to the other station's speed, so I am 
surprised at
>
>his experience. Please encourage him to give it another try.
>
>On the antenna front, a G5RV is certainly not the best option and 
the 
>radiation angle of a vertical is probably worse (in most "skip" 
conditions).
>
>An 80 loop, longwire, or 80m dipole would be better.
>
>I would rather use a "bent" 133ft dipole at 10ft than a G5RV at 40ft!
>
>73, Rob
>G4LMW
>http://www.G4LMW.co.uk
>
>
>
>----- Original Message ----- 
>From: "Danny Higgins" <danny.higgins at keme.co.uk>
>To: "'Thomas Cannon'" <tom.g0vqr at ntlworld.com>; "'UK Contesting'" 
><uk-contest at contesting.com>
>Sent: Wednesday, April 21, 2010 9:51 PM
>Subject: Re: [UK-CONTEST] QRS and QRP
>
>
>> Tom,
>>
>> I always slow down if I am called by a QRS station because I want 
to make
>> sure he gets the call and the report correct.  Also, he may come 
back 
>> again
>> in the next contest.
>>
>> I use QRP for CW and DATA when conditions are good, as 100W to 10W 
is the
>> same as S9+30 to S9+20.  I use a full size 80M dipole up about 40 
feet and
>> in the last CW contest I managed to hold a run frequency for over 
an hour.
>> It helps to have a low noise level to pick up the other QRP 
stations who
>> call.  I'll probably be QRP in tomorrow's data contest as I can 
only run 
>> 50W
>> in data modes, so only disadvantaged by 7dB this time.
>>
>> Danny, G3XVR
>>
>> -----Original Message-----
>> From: uk-contest-bounces at contesting.com
>> [mailto:uk-contest-bounces at contesting.com] On Behalf Of Thomas 
Cannon
>> Sent: 21 April 2010 21:14
>> To: UK Contesting
>> Subject: [UK-CONTEST] QRS and QRP
>>
>> I was approached by one of the members at are Radio club the other 
night 
>> who
>> has just started to go into the Club Championship contests. He has 
never
>> been in Amateur Radio contests before so was not sure of the QSO 
format. 
>> He
>> also is learning Morse Code and was keen to try it out in the 
contest.
>> I explained the use of cut numbers in contest N - 9 and T - 0 and 
the
>> contest QSO format. which he was fine with.
>> The problem he had this month in the CW  leg was that only 1 
station out 
>> of
>> the handful he worked slowed down so he could understand them. I 
have been
>> trying so hard this year to get members to take part in the Club
>> Championships especially the members that are not contesters and 
the ones
>> learning Morse Code.
>> His comment to me was:" I am not sure I will do that again, only 
one 
>> person
>> would slow down". If we want newcomers to take part in these 
contest we 
>> must
>> slow down to their speed. I have convinced him to give it another 
try, so
>> please if you are sent Morse very slowly, please send it back 
slowly to 
>> the
>> other station.
>>
>> Also congratulations to some of the QRP stations in the Club 
>> Championships.
>> There were some big scoring logs and I wish I had a station as good 
as
>> theirs! I think the antenna must be the most important part of the 
>> station.
>> I use a full size G5RV at about 30 feet and when the weather 
permits I use
>
>> a
>> 40 metre verticle on a Helikite with 16 - 20 Metre ground radials. 
If 
>> anyone
>> has any better ideas for an antenna on 80 Metres I would love to 
hear 
>> about
>> it.
>>
>> Work you in a contest soon
>>
>> Tom G0VQR
>>
>>
>>
>> _______________________________________________
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