----- Original Message -----
From: "Brian Machesney" <nekvtster@gmail.com>
To: <topband@contesting.com>
Sent: Friday, March 27, 2009 10:31 AM
Subject: Topband: Tower-attached guy wires - max length for driven tower?
> I'm putting up 80 ft of Rohn 25G with various antennas on top and may
> decide
> to drive it as a monopole on 80 and/or 160 at some time in the future.
> Since
> stubs of wire attached to a radiator can affect its behavior, is there a
> recommendation as to the maximum length of the guy wires whose ends are
> attached to the tower? While the most conservative method might be to use
> an
> insulating guy material for this length, I would like to use the 1/4" EHS
> I
> have on hand instead.
>
> Thanks,
>
> Brian K1LI
---------------------------------------------------------------------------------
Brian,
>From the ARRL Antenna Book the chart shows that the longest length that is
non-resonant in six main MF & HF amateur bands is 57 feet. If you are
concerned about the three WARC bands then you will have to select some
shorter length according to the chart. 39' clears all the bands except 24
mHz for example.
I have constructed numerous guyed towers using this formula and I have never
had any guy wire interaction with resonant antennas. By using this length
you get maximum length of guy wire between insulators resulting in less
expense for insulators and attachments. Insulate your guy wires at the
tower and then at 57', and then at additional 57' lengths as necessary.
One structure I put up was 90' of Rohn 25 above an insulated base at 40'
above ground. The loading of a Force 12 C4XL at the top of the 90' puts the
resonant frequency of the 160 Meter Ground Plane at approximately 1.870 mHz.
By adding a 10' pipe stinger above the C4XL, the resonant frequency is in
the range of 1.840.
You can go to http://www.eaars.org/~n7gp/ and see a couple of photos of the
antenna system.
In this case there are more than a dozen 1/4 WL elevated radials. In your
case you could lay traditional radials and shunt feed the tower, or you
could install the tower on an insulated base and series feed it.
Good luck with the project.
73 de Milt, N5IA
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|