Re: [TowerTalk] Re: One more ground radial question

[Jan Erik Holm <sm2ekm@telia.com>]

Comments below:

corneliuspaul@gmx.net wrote:

> In my opinion, Jerrys question was best answered by N0AH.
> He describes exactly what any vertical/inv L builder will find out eventually:
>
> The more radials you put on - the worse the SWR gets.
> This is because ideally the antenna has a feedpoint impedance of 10 - 30 Ohms.
> But, when you have only a few radials, this value gets "covered up" by the
> lossy earth network, so what your coax and transmitter sees is much more
> near 50 Ohms which makes you think its all fine. Another reminder that good
> SWR does not necessarily mean good antenna. It cant be repeated often
> enough: remember, the best SWR you can get is from a dummy load.
> What you need with more radials is either an UNUN or a matching network or
> a tuner (ideally located at the feedpoint).
Lossy earth network, no offence but that was something new to me.
Why not explain like this: feedpoint Z consists of two components,
Radiation resistance and Loss resistance, R-rad we cant do much
about but R-loss we can make smaller up to a certain point with more
and more radials in the near field.
> Now on to the number of radials:
> I find it relieving to finally see those come out of the woods who DONT
> always use 60 or 120 radials (e.g. N6RK).
> For me its amazing to see the number of posts demanding such a number of
> radials as "sufficient".
> Amazing, because remember we are speaking about 160m radials here.
> If you have a wide space available and can simply put them on the ground -
> well then its a feasible task but if you have a city lot or whatever, you
> will find it difficult enough to put on 4 or 8. 
> But - by all means, DO IT and have fun on the band! Your antenna will still
> work much better than you might expect from the statements of those who
> demand 60 radials.
> When going on expedition or portable operation I have put up an 80m/160m
> vertical/inv L  (using one 80m trap) several times. 
> I found a practical set of radials consists of 4 radials cut for 160m (40m
> long) and 8 radials cut for 80m (20m long). 
> Yes of course, more is better, but I said "practical". I am always trying
> to bring at least 2 radial sets (8 for 160m, 16 for 80m) but I either leave
> them at home for cutting down the weight or I drag them along and then dont
> find the time / space to put them to work... :-)
Expression practical is kind a fishy to me. What´s practical to some isn´t
practical to others.
Why not look at facts instead. Facts are that "around 120" 1/4 wave radials
will give you very close to no R-loss at all.
Around 60 1/4 wave radials you are looking at "around" 1 dB of R-loss and so on.
Now there are variations on that, like if you just can put a small number
of radials you get less R-loss if you make them shorther then 1/4 wave, and
if you use only 2 or 4 radials it´s better to elevate them, however everything
like that will be "dB´s down" from the case 60 1/4 wave on the ground.
Years ago W8JI did some measurements that might be found somewhere I guess,
his measurements seemed very correct.
Bottom line, when someone asks me how many radials to put out I always
say "as much as you can" or put out 60 then you will have a loss in the near
field that is resonable.
Feedpoint Z has never been an issue to me on vertical antennas. If I put up
a vertical I will try to minimize the loss in the near field as much as I can,
whatever feedpoint Z I end up with I just match to my coax, usually I just
use a capacitor or two and a stub since it´s easy to make sutch a matching
network that will sustain high power, UNUN´s and gizmos like that I stay
away from, the simpler the better!
> Good luck!
> mri xmas and hny 2004
>
> 73
> Con DF4SA
73 and seasons greetings, maybe Santa will bring you some radials
Jim SM2EKM




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