[Antennas] G5RV

[Robert Lay]

Dear Durwydd,

I would suggest that there are no formulas that will be of help in doing
what you are suggesting.

Instead, I would suggest using a straightforward, classic "folded dipole"
antenna fed with you home brew open wire line.

I would also recommend that you abandon the coax and the bead isolator
concept in that it may be effective at times with some configurations but
not consistent or workable with other configurations and frequencies. A much
more satisfactory solution for the transmit end of the feedline is a proper
balanced tuner, and lacking that, use a heavy duty balun with a short run of
coax to an transmatch CLC tuner.

Such a configuration will almost always be tunable as an entire system. (I
use such an arrangement on 40 m and up with a nominal 40 m folded dipole.

There is no need to be concerned about the impedance seen at the feedpoint
of the dipole, nor is there any concern about the characteristic impedance
of the open wire line. All of that is academic and will work because the
stresses (current and voltage) will still be well within you materials
capabilities and losses will be very minimal. It would only be in the tuner
itself and the coax segment and balun (if present) where any significant
losses would occur.

Minimizing the losses in any balun and coax segment is a matter of making
the impedance seen looking into the balanced line conform more or less to
that expected by the balun and coax. For example, if the balun is 4:1
(impedance ratio) and the coax is 50 ohm, then the balun is going to match a
200 ohm balanced line. Since your line is not likely to present exactly 200
ohms at ANY frequency, you are obliged to "help" it out a little with
corrective reactances. Either a parallel capacitor, two series capacitors, a
parallel inductor  or two series inductors are often very helpful in
reducing the mismatch such that stresses on the balun/coax segment are
reduced and the tuner's job made much easier.

Knowing what sort of reactive compensation is needed is not an art nor is it
magic, it is either trial and error or you have access to an impedance
measuring device. I am fortunate in having both an expensive General Radio
RF Impedance Bridge and the MFJ Antenna Analyzer - either of which allows a
measurement of the impedance and application of necessary corrections.

For the cheaper correction method, you simply apply additional length of
transmission line, such as advocated by Cecil Moore (see his web site) or
you can get really sophisticated and use parallel stubs fabricated from
segments of your open wire line.

Again, be advised that there are no pre-established formulas for this - you
simply have to apply theory to the problem and tailor your configuration to
the situation. In the process you will learn what works for you, and that is
all that matters. A good A/B switch and a second antenna are invaluable in
making final decisions as to which of two antennas is the better in a given
situation.

Bob Lay in Dahlgren, VA
http://www.qsl.net/w9dmk
----- Original Message -----
From: "Durwydd MacTara" <durwydd@hotmail.com>
To: <Antennas@mailman.qth.net>
Sent: Thursday, July 17, 2003 12:39 AM
Subject: [Antennas] G5RV


> For a limited space application I want to "double back" the arms of a G5RV
> shorty flat top using pvc pipe for spacers. will this work? how much
spacing
> should I use?
>
>   "Communications" is a People skill, the rest is technology.
>              Durwydd MacTara
>
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