[TowerTalk] TowerTalk Digest, Vol 33, Issue 7

[Dudley Chapman]

Vojtech wrote and Jim Lux responded......

Message: 8
Date: Thu, 01 Sep 2005 15:52:31 -0700
From: Jim Lux <jimlux at earthlink.net>
Subject: Re: [TowerTalk] 4SQ for 80M
To: Vojtech Ritzko <om8aw at yahoo.com>, towertalk at contesting.com
Message-ID: <6.1.1.1.2.20050901154431.080eb1c8 at mail.earthlink.net>
Content-Type: text/plain; charset="iso-8859-1"; format=flowed

At 01:35 PM 9/1/2005, Vojtech Ritzko wrote:

>I am planning 4SQ for 80M. I have problem with space, due to property 
>limitation.
>
>
>
>Does anybody compared:
>
>
>
>4SQ with ?/4 spacing, but only ?/8 long radials
>
>
>
>Versus
>
>
>
>4SQ with ?/8 spacing with ?/4 long radials
>
>
>
>TNX
>
>OM8AW

I'll give you a theoretical take.  Others will no doubt give you some 
practical experience.

Closer spacing makes for higher interelement coupling, the feedpoint 
impedances will probably be lower, so the IR losses (assuming everything 
else is identical) will be higher. The resonance will be higher Q, and the 
coupling changes faster with frequency, so it will be harder to get good 
phasing. Some depends on: would you use lambda/8 phasing or lambda/4 
phasing (or some variant with a bit more phase shift than the physical 
distance (like W8JIs scheme), which increases the forward gain a bit).

Think of something like a W8JK beam (two parallel dipoles fed out of 
phase).  As you move the two closer together, the directivity increases, 
but so does the loss, so the overall gain doesn't increase all that much.

On the other hand, the length of the radials (assuming you're planning on 
putting them on the ground) isn't really frequency dependent (the 
propagation speed in the wire is much slower than in free space, so a 
quarter wavelength long wire is a much larger fraction of a wavelength hwen 
the wire is buried in, or on, the dirt).  Most of the current in the 
antenna is close to the base, and it's the same for the radial 
network.  Lots of wires close to the base is probably more important than 
few wires farther out.

If I had the choice, I'd go for 1/4 wave spacing and short radials.  Easier 
tuning and operation, mostly.

....................................................................

Vojtech,

I cast my vote with Jim for the very same reasons.  Radials of 1/8
wavelength in the ground are very effective as long as there are many of
them.  In fact, any length beyond 0.2 wavelength is not worth the effort
even if you have the room. Here is a link to a forum conversation that shows
a good rule for determining the length and number of radials vs the
efficiency.

http://qrp.kd4ab.org/2001/010326/0041.html

Dudley - WA1X