[TowerTalk] Re: Radials over salt water

[Michael Tope]

Heres one for all you radial efficiandos. I am involved with a site (workplace club 
station) where the antennas sit near the edge of a 400 foot mesa. The mesa drops 
off in the direction of our longpath heading to Europe suggesting that the site should 
be favorable for this path. On the air experience on 80 meters suggests that we do 
"okay" compared to other well equipped stations in the area, but my gut feeling is 
that there is significant room for improvement. The 80 meter transmit antenna is a full 
wave delta loop hung off a 60 foot tower and fed at the bottom corner. The bottom leg 
of the delta loop is only about 8 to 10 feet above ground. The antenna is probably 
about 100' from edge of the mesa (the terrain drops at > 45 deg beyond this point). On 
160 meters, we are using a inverted vee which is on another tower (telephone pole with 
metal mast extension). The apex is  about 50 feet above the ground with the antenna 
broadside to the longpath heading. This tower is even closer to the edge of the mesa 
with the base being roughly 50' from the "cliff". 

I have been trying to lobby my colleague who built all this stuff that we would 
probably do better on transmit using a vertical with an extensive ground system, but
because of the logistic difficulties involved with doing this, he has been somewhat 
resistant to my suggestions arguing instead that we do pretty well already and that
the 80 meter delta loop is fed at a point that is optimum for long angle radiation. 
My argument is that delta loop is probably inefficient since at least one of the current 
maxima's is close to the ground and there are no radials under the antenna. For 160
meters, I am wondering what is the effective height of the inverted vee antenna? From
a far field perspective its probably 400 to 500 feet above the surrounding terrain (at
least to the south), but in terms of near field coupling, its very close to the ground 
immediately underneath it, so I would expect a lot of induced current that would 
decrease efficiency, and also possibly negatively influence the low angle performance
of the antena towards the south (I would expect the induced ground current to be 
out-of-phase with the antenna current). 

Anyone have any suggestions. Would it be worth the trouble to switch over to 
verticals and start adding ground radials?

Thanks,

Mike, W4EF.........................
 
----- Original Message ----- 
From: "Maurizio Panicara" <i4jmy@iol.it>
To: "Bill Coleman" <aa4lr@arrl.net>; <W8JI@contesting.com>; <towertalk@contesting.com>; <K3BU@aol.com>
Sent: Thursday, January 11, 2001 10:53 AM
Subject: Re: [TowerTalk] Re: Radials over salt water


> Space between radials has to be equal or less than .025 WL.
> The tables consider .5WL radials and this numbers lead to the famous 120
> radials (>90% efficiency).
> Unless the radiator is longer than 1/4 WL there's no need to extend radials
> to .4-.5 WL.
> Halving the radials lenght and keeping the same .025 distance, the required
> number for .025 spacing is 60 .
> In theory and in case of a full size quarter wave radiator, with 30 radials
> the efficiency drop from 90% to 86% that's an attenuation of 0.25 dB if
> compared with the 60 radials case.
> 
> 73,
> Mauri I4JMY
> 
> ----- Original Message -----
> From: "Bill Coleman" <aa4lr@arrl.net>
> To: <W8JI@contesting.com>; <towertalk@contesting.com>; <K3BU@aol.com>
> Sent: Thursday, January 11, 2001 4:39 PM
> Subject: Re: [TowerTalk] Re: Radials over salt water
> 
> 
> > On 1/10/01 15:35, Tom Rauch at w8ji@contesting.com wrote:
> >
> > >> Thinking and analyzing the situation, there should
> > >> be improvement if using many (>60) elevated (or on ground) radials vs.
> few
> > >
> > >Fact Yuri. When the radials are less than .025 to .05  wl apart at
> > >the open ends, they look like a solid screen. Using more radials
> > >than that is a waste of wire.
> >
> > Hmm. 1/4 wave radials means a circle of 1/2 diameter. That's a perimeter
> > of pi/2 wavelength, which divided by .05 yeilds about 31 radials.
> > Similarly, .025 spacing at the ends is about 63 radials.
> >
> > If >63 radials is like a solid screen, I wonder why there's so much in
> > the amatuer literature about installations with 120 or so radials.
> >
> > Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
> > Quote: "Not within a thousand years will man ever fly!"
> >             -- Wilbur Wright, 1901
> >
> >
> > --
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> >
> 
> 
> --
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> 


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