Let's keep in mind that radials in physical contact with the earth are
essentially non-resonant; their length is of importance only to the extent
that collectively they form an effective ground screen to collect return or
displacement current to the feedline inner braid. Their action is not to
serve as a reflector of sorts to provide some for the waves to "bounce
against" in the process of being launched. That occurs many wavelengths away
in the confluence of the Fresnel and far-field zones and defines the takeoff
angle involved in the vertical radiation pattern.
I use eighteen 25 ft radials with a Butternut HF-9V simply because (a) that
is about the height of the antenna and (b) I made a good buy on 450 feet of
insulated #12 green wire at Lowe's which translates into 18 twenty-five foot
radials. The wire size is massive overkill, but it was cheap!
Jerry Sevick demonstrated in the 70's that 0.2 wavelength radials are
essentially as effective as anything longer and a rule of thumb that radials
as long as the vertical is high works out pretty well.
Elevated radials are an altogether different story. There you want (a)
resonance and (b) a balanced configuration so that the radiation among the
radials is cancelled as much as possible. So symmetry is important.
Preferably one would tune a pair or two of radials for each band in use.
However, ground-mounted radials are largely frequency independent and the
aim there is just to get as much copper in the immediate vicinity of the
feedpoint as is feasible. More shorter radials are preferable to fewer
longer radials, according to Jerry's extensive experiments. All this is
documented in his book on "ununs" and "baluns." As well as in his original
series in QST.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>
----- Original Message -----
From: "Stuart Rohre" <rohre@arlut.utexas.edu>
To: <tentec@contesting.com>
Sent: Wednesday, April 02, 2003 3:56 PM
Subject: Re: [TenTec] an antenna question
> Yes, the transmatch should handle that OK.
> The radials are for a lower band and will function as a ground screen does
> to an extent on higher bands.
> 73,
> Stuart K5KVH
>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
|