Greetings from Chiangmai
where every prospect pleases.
Been following the thread started by the gentleman asking about feeding his
G5RV (or variant thereof).
The humble G5RV antenna was invented not long after WWII by the
multitalented Louis Varney (SK), intended as a cheap and cheerful compact
all hf band antenna. In those days the hf bands were 10, 20, 40 and 80m. The
15m band had not yet been opened to the amateur service and the WARC bands
were not even a gleam in the radio amateur's eye.
Basically a dipole (or inverted V) of some 100 ft length (the UK had yet to
be metricated) centre fed with a balanced feeder of around 33 ft and thence
by 75 ohm coax of length sufficient to reach the transmitter.
If the thing was unravelled it approximated a dipole of 80ft per side. The
magic 80 ft length gave an impedance at the coax junction that was not 75
ohms with zero reactance, but a value of R +jx which was not too horrible on
most of bands.
So with a bit of luck and the wind in the right direction the coax could be
connected to the transmitter of the day which like as not was home built and
had the final tank coil link coupled to the antenna, or if one of the new
fangled pi couplers was intended to cope with a range of R+jx that a modern
all singing all dancing solid state transceiver would refuse to deal with
except via an ATU.
Once a generation of amateurs who had happily used open wire feeders and who
gave not a damn about SWR had gone to the great ham shack in the sky, or
converted to txs in tin boxes that had coax connectors on the output, things
changed.
Low SWR became all the rage. Gentlemen even wrote learned articles reporting
studies where G5RV like dipoles had their dimensions tweaked in an endeavour
to get something like 50 ohms non reactive at the feedpoint. 15m was always
a problem but, hey, the thing had never been meant to work on 15.
Bit like the search for the Philosopher's Stone or the Lost Ark.
Seems like this tradition is alive and well today right here on Tower Talk.
My advice:
Relax!
Build the G5RV to a reasonable approximation of the original design. If you
want a balun wind say 10 turns of the coax around a 100mm/ 4 inch plastic
pipe before hooking it on to the bottom of the balanced line. Back in the
shack , see what you get. If necessary use an ATU to get the antenna to suck
power.
Then enjoy amateur radio. Chances are you will be pleasantly surprised by
the performance of this modest antenna.
73 es gl
Barry ZL1DD
--
Barry Kirkwood PhD ZL1DD
barry.kirkwood@gmail.com
NZ: 021 160 8999
Thailand 081 235 1556
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