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Author Topic: Building G5RV dipole for 80-10 M Have ?s about feedline, choke, ladderline, coax  (Read 1337 times)
AB3OL
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Posts: 18




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« on: March 06, 2013, 03:25:31 PM »

I would like to construct a 80-10M G5RV dipole and feed it from my TenTec Omni C 546C transceiver. Have a MFJ-949E Versa Tuner.
From the various sites on the internet I see that the two antenna segments should be 102' (or 51' each) from a center fed insulator. Most diagrams show 30' of ladder line and then choke consisting of 18' to 21" of 50 ohm coax then more coax to the transmitter. I have a 28' section of thick ladder line (think its 450 ohm) and have both RG8U and RG58U coax. Can I just connect the 28' of ladder line to the coax with the choke described above then feed say 70' of coax from there to the MFJ tuner and them to the transmitter. I know the MFJ tuner can feed the ladder line using its internal 4:1 BALUN but I do not have a good way to run the ladder line directly into my house. I would prefer to run coax to the ladder line and make a splice outside.
Do I need 30" of ladder line or is the 28' section I have sufficient?
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W0BTU
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« Reply #1 on: March 06, 2013, 04:28:53 PM »

In the first place, there is no such thing as a G5RV antenna (or derivative thereof) that efficiently covers all bands from 80 through 10 meters. Even with a tuner Smiley  See http://www.w0btu.com/g5rv_antenna.html

Among other things, the length of the antenna, the length of the feedline, and the impedance of the feedline are all somewhat interactive, in order to have the G5RV work right on more than one band.
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AC2EU
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« Reply #2 on: March 06, 2013, 07:02:48 PM »

If you want multi-band operation that works, perhaps consider a fan dipole for bands of interest.
They are best in warmer climates. They don't do well with snow/ice.
However just about anything would be better than a G5RV, IMO.
see also:
http://www.karinya.net/g3txq/g5rv/
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WB6BYU
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Posts: 13580




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« Reply #3 on: March 07, 2013, 12:29:21 PM »

Quote from: AB3OL
I would like to construct a 80-10M G5RV dipole...



Then I strongly recommend you review VK1OD's articles on the topic:

http://vk1od.net/antenna/G5RV/
http://vk1od.net/antenna/G5RV/optimising.htm

Whether such an antenna actually covers all the ham bands between
80m and 10m depends on the losses you are willing to accept and the
tuner you have available to use.  For example, many hams use them
on 10m, even though the SWR is high and you may have 5 or 6dB of
loss in the coax (depending on type and length).  That's 3/4 of your
power lost in the coax, and about 1 S-unit lower signals on both ends,
but you can still make lots of contacts when the band is open.


Quote

Can I just connect the 28' of ladder line to the coax...


That will shift the SWR curve somewhat.  You can bring it back in for one
band by readjusting the wire lengths, but the original design is a carefully
crafted combination of lengths.  If you have to, make up an extra 2' of
open wire line and stick that at the feedpoint before switching to the
ladder line.  (It does also depend on the style of ladder line - if it has a
continuous insulation without windows, then the velocity factor might be
high enough to allow a 28' length to work.)

The primary cause of losses (at least above 80m) is the operation of the
coax at a high SWR.  The shorter the coax, and the lower the loss per foot,
means a higher percentage of your transmitter power gets radiated.


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W4VKU
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Posts: 358




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« Reply #4 on: March 07, 2013, 04:38:50 PM »

I have a 80m G5RV with 300 ohm ladder line and a coax choke built out of LMR400 in use for over 5 years.
The choke was just 8 turns of the same coax line and it continues to the shack.
Prior to changing to LMR400, i used a certain length of RG213 and i had fantastic match on 20m and 40m,
but so-so on 80m.

When it rained, the swr on 80m would be good, but not so on 20m.

Later on, when i swapped out the RG213 with LMR400(not sure of the length), but the swap shifted
the SWR and now 80m is perfect for cw and so-so for 75m. It is off on 40m/20m. But we have gain
antennae for those bands and don't care any more.

We prefer not to use tuners, but when it rains and i need to work 80m, i had no choice, but to
bring the Palstar inline.

When it was resonant on 20m, it worked real well. However, it is a doublet on 20m and had nulls into
parts of the persian gulf.  It never worked well on 10m.

I looked into performing  Mods that allowed it to work on 17m etc, but never got to it.

Like the other suggestion, a fan dipole might be a good way to go, provided there is patience to prune
and handle the interaction.

Also, a G5RV is very touchy with a tuner. Once you have tuned the tuner and you shift frequency,
it might need a retune. My MFJ-986 did not like it , but the MFJ-949E did tune it nice. The palstar
does a mighty fine job these days.

what ever you decide, experiment, learn and have fun

73s
krish
w4vku
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