Discussion:
G5RV length of coax
(too old to reply)
SimonC
22 years ago
Permalink
I am receiving conflicting advice as to whether the length of coax attached
to the twinlead is significant. If the length of the coax is important
(other than being long enough to reach the radio!) please let me know....

Someone has told me that 45ft (and multiples thereof) is the correct length
to aim for.... Others have said 70ft.....
Thierry
22 years ago
Permalink
Here is some more advice... that can helps.
http://www.astrosurf.com/lombry/qsl-g5rv-2.htm

Thierry
ON4SKY, LX3SKY
Post by SimonC
I am receiving conflicting advice as to whether the length of coax attached
to the twinlead is significant. If the length of the coax is important
(other than being long enough to reach the radio!) please let me know....
Someone has told me that 45ft (and multiples thereof) is the correct length
to aim for.... Others have said 70ft.....
Reg Edwards
22 years ago
Permalink
Change your advisers. You don't need any coax at all.

Bring the twin line all the way back to the shack. You will then have a MUCH
better multiband antenna.

The G5RV was designed with its 1/2-wavelength open-wire feedline, without
any coax, for 14.15 MHz only.

14.5 MHz is the only frequency at which coax of no special length is OK.
Prefer 75 ohms. For that frequency only you may not need a tuner.

Otherwise, if you insist on multi-band AND coax, you can use any old length
or impedance you happen to have lying around the shack without noticing any
difference from the poorer performance.

The best of all multi-band, multi-directional dipoles for YOUR QTH has a
no-special length which fits nicely into your backyard and has a 450 or
600-ohm feedline all the way to the shack with a choke balun between
feedline and tuner.

You can have ANY convenient length of 50-ohm coax between tuner and
transmitter.

Avoid coax in feedlines like the plague for HF multiband working unless you
suspend a remote-controlled tuner in the middle of the dipole.
---
Reg, G4FGQ
Nick G4FAL
22 years ago
Permalink
Post by Reg Edwards
Change your advisers. You don't need any coax at all.
Bring the twin line all the way back to the shack. You will then have a MUCH
better multiband antenna.
In my experience this depends where the aerial is in relation to your radio.
In my case my transmitter is in the attic of the house which is near to the
end of the dipole. I have found that running balanced feeder parallel to the
dipole leads to problems with radiation from the feeder.

I have used two solutions to this which I am sure Reg will object to.

1. Use a doublet centre-fed with balanced (450 ohm) ribbon feeder. This
feeder should drop directly perpendicular from the dipole. Use a 4:1 balun
at ground level and then low loss coax to feed your ATU. Experiment with
lengths of dipole, balanced feeder and coax to best effect. This
configuration is good for temporary aerials as the ribbon is light in weight
and the dipole part can be held up with sea-fishing line between trees. Coax
can trail across the ground which is not a good idea with balanced feeder.

2. Use parallel dipoles with a centre balun and fed with low-loss coax. I
have 80 and 40 metre dipoles in this configuration which gives me resonant
aerials for 80m, 40m and almost 15m. As a desperate measure this can be
'made' to radiate on 5MHz, 10.1MHz, etc.

Incidentally, if you search for "g5rv coax feeder" in yahoo.com you will
find plenty of advice on length of coax to attach to your G5RV feeder - 70
foot seems to crop up quite often. If in the unlikely event that you have a
balanced tuner and your radio equipment is conveniently situated in relation
to the dipole elements then you will be better off without the coax.

Simon, if this is another G5RV wind-up troll then congratulations, we have
fallen for it again.


--
Nick G4FAL
http://www.totterdell.co.uk
Reg Edwards
22 years ago
Permalink
Post by Nick G4FAL
I have used two solutions to this which I am sure Reg will object to.
===============================

Nick, goodness gracious, why should I object? They appear to be logical
engineering solutions. Your only mistake was starting in the first place
with a G5RV (if that's what you did) with its wicked length of coax. No
wonder it can be operated very occasionally on more than just the 20m band
(for which Mr Varney originally designed it) without a tuner.


In the past, having been in the same siuation myself, with the end of a
random-length dipole (which is what a G5RV is) not far from the bedroom
shack, the problem was solved by attaching the near end of the dipole to a
tall (TV) mast mounted on the house chimney. The other end of the dipole was
suspended from a 30-foot timber mast near the other end of the garden until
a teenage vandal hacked into one of the three clothes-line guy ropes just to
hear the terrific shuddering crash.


The dipole was fed at its near end over a single-wire, 600-ohm, transmission
line which descended dog's leg fashion down to window level. It was secured
in place by jamming it in the closed window. As it was a metal window frame
it was necessary to insulate the feedline with a few inches of electrician's
tape. The feedline then continued 3 or 4 feet direct to the tuner and
transmitter. I understand an end-fed dipole, when fed via a more-or-less
vertical single-wire transmission line, is often described as an
"Inverted-L". Or in a more dignified manner by referring to it as a
"Marconi".


Of course, there's radiation from the feedline but with a multi-directional
dipole anyway, there's nothing wasted. I had one case of TVI. But that was
due to the next-door neighbour's TV-repairer fogetting to replace the IF
amplifier's screening can. Easily fixed. The neighbour was a dentist but I
always used somebody who lived further away.


The tuner consisted of a tapped coil, close-wound with 20-gauge enamelled
wire on an empty toilet roll tube, a variable 500pF receiving-type
capacitor, a pile of various-length connecting leads terminated with mini
crocodile clips, and then to the home-brew transmitter which was eventually
upgraded to a TS-520 and finally modified to death.


As you know, with an Inverted-L, a modest ground connection is needed. This
was provided by drilling a hole in the bedroom floor boards and extending a
copper braid down to the domestic incoming water-main lead pipe with its
water meter, plus the substantial sheath of the incoming 50Hz power supply
cable. Overall length, including ground ground connection, was about 0.3
wavelengths on 160m.


These conveniently located ground connections (in the garage) were connected
to thousands of miles of countless numbers of radials buried under our Great
but now Deprived City of Birmingham. (For they who are unfamiliar with world
geography - it's the original B'ham.) Using the radiating and ground
system desribed above I have indeed worked into the other B'ham which
according to the song is situated "Way Down South in Alabam". The QSL card
was lost during a house move.
----
Reg, G4FGQ
RVMJ
22 years ago
Permalink
Post by Reg Edwards
Using the radiating and ground
system desribed above I have indeed worked into the other B'ham which
according to the song is situated "Way Down South in Alabam". The QSL card
was lost during a house move.
----
I didn't know you were a fan of Lynyrd Skynyrd.

I once nearly went in for a job in Huntsville....just think of the
music.....
--
from
RVMJ
(at) Bigfoot (dot) com
Walt Davidson
22 years ago
Permalink
Post by RVMJ
Post by Reg Edwards
Using the radiating and ground
system desribed above I have indeed worked into the other B'ham which
according to the song is situated "Way Down South in Alabam". The QSL card
was lost during a house move.
I didn't know you were a fan of Lynyrd Skynyrd.
"Well, I ride to Alabama
With my pistols out by my side.
Well I ride to Alabama
With my pistols out by my side.
Cause down in Alabama you can run
But you sure can't hide."

"Mississippi Kid", Lynyrd Skynyrd, 1973
(by the way)

73 de G3NYY
--
Walt Davidson Email: g3nyy @despammed.com
Martin
22 years ago
Permalink
Post by SimonC
I am receiving conflicting advice as to whether the length of coax attached
to the twinlead is significant. If the length of the coax is important
(other than being long enough to reach the radio!) please let me know....
Someone has told me that 45ft (and multiples thereof) is the correct length
to aim for.... Others have said 70ft.....
It certainly does make a difference - as I discovered recently when I made
up a new cable to connect from the wall-mounted socket to the rig and
increased the length by a few feet - the tuner settings changed
significantly on most bands. I'm not sure that there is any "correct" length
(other than zero as many will point out) - but I guess from my experience
that there will be lengths which effectively make some frequencies virtually
impossible to match.

Martin (G8FXC)
Andy Cowley
22 years ago
Permalink
Post by SimonC
I am receiving conflicting advice as to whether the length of coax attached
to the twinlead is significant. If the length of the coax is important
(other than being long enough to reach the radio!) please let me know....
Someone has told me that 45ft (and multiples thereof) is the correct length
to aim for.... Others have said 70ft.....
The correct length is 0. Use twin all the way to the ATU.


vy 73

Andy, M1EBV
Nick G4FAL
22 years ago
Permalink
Post by Andy Cowley
Post by SimonC
I am receiving conflicting advice as to whether the length of coax attached
to the twinlead is significant. If the length of the coax is important
(other than being long enough to reach the radio!) please let me know....
Someone has told me that 45ft (and multiples thereof) is the correct length
to aim for.... Others have said 70ft.....
The correct length is 0. Use twin all the way to the ATU.
Andy

You must have been busy gaining so much practical experience of G5RVs these
past three months ;-)

Incidentally the MFJ manual at
http://www.mfjenterprises.com/man/pdf/MFJ-949E.pdf had some good tips for
random sized doublets with twin feed on page 8 - fails to mention the
velocity factor but otherwise useful. Unlike Reg's standard response, there
are sizes that are better than the biggest for your garden and some lengths
of feeder are more appropriate than others.

--
Nick G4FAL
http://www.totterdell.co.uk
Thierry
22 years ago
Permalink
Post by Nick G4FAL
Incidentally the MFJ manual at
http://www.mfjenterprises.com/man/pdf/MFJ-949E.pdf had some good tips for
random sized doublets with twin feed on page 8 - fails to mention the
velocity factor but otherwise useful.
True, the velicity factor has to be taken in account to calculate the lenght
of your feeder line.
See commentaries on http://www.astrosurf.com/lombry/qsl-g5rv-2.htm,
paragraph "Machting section".

Thierry
ON4SKY, LX3SKY


Unlike Reg's standard response, there
Post by Nick G4FAL
are sizes that are better than the biggest for your garden and some lengths
of feeder are more appropriate than others.
--
Nick G4FAL
http://www.totterdell.co.uk
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