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G5RV Antenna Question

May 22nd 2016, 20:45

N5ERV

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I have been an inactive ham since the late 70s/early 80s. I would like to install a MFJ-1778 G5RV antenna from a 30' Rohn 25-G antenna tower. The former owner of this home had a TV antenna on the tower. It is mounted in/on a concrete base but has never been guyed.

The antenna instructions say the "antenna must be more than 35" above ground level to give acceptable performance, and should be as high as 80 ' for the best overall performance."

Given that information, I'm thinking I should add 1 or 2 more 10' sections to the tower, taking it up to 40' or 50'. It I do that, I'll have to guy the 40' or 50' tower and probably have to rework the concrete base.

Do any of you have experience with the G5RV antenna and have any recommendations about tower height, etc.? I do not want to go above 50' in tower height. Depending on the antenna orientation I decide on, the ends of the 51' antenna sections will probably be terminated in nearby trees.
May 24th 2016, 00:00

WA0CBW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I think you might find those instruction are applicable to most dipoles including the G5RV. Extending your tower to 40-50 feet may be more trouble than it is worth. I would try installing the G5RV on a 5 to 10 foot mast on your existing tower and see how it performs.

BB
May 25th 2016, 13:44

N5ERV

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I won't have an opportunity to install a 5' or 10' mast on top of the tower since I plan to install a Comet GP-6 dual band vertical (2 meter/70 centimeter) on top. I need to determine how much wind load the GP-6 and G5RV will add to the tower and then evaluate placement of some guying to the 40' tower.
May 25th 2016, 17:29

NV2K

Joined: May 20th 2014, 01:06
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Total Posts: 0
Mounting it 80 feet high should provide you with good DX performance on 80m, and very good to excellent DX performance on higher bands.

You should have very good performance on 20m if you mount it as an inverted V with the apex 30 feet above ground, particularly if you get the ends at least 10 feet up. It's intended to be used as a flat-top doublet on 14Mhz, so you may need use a tuner if you're not comfortable with adjusting the length of the stub. But to use it on other bands, you'd want to use a tuner, anyway.

On 40m the apex would still be almost a quarter wavelength up, so it should still work reasonably well on 7MHz. On 80m, the antenna will be short (large capacitive component with low radiation resistance) and low (making it something of a cloud-warmer), but with a tuner you should be able to work local contacts. It probably won't be sufficient to get you your 80m endorsement for WAS, but you should still be able to work contacts on 80m (and you can expect to do much better on the higher bands).

Good luck!
Apr 7th 2017, 00:51

K6EB

Joined: Jan 19th 2017, 17:49
Total Topics: 0
Total Posts: 0
I'm using the MFJ G5RV jr. (half as long). I only can get it up 18 feet, but it is in the open clear of anything metal. I am amazed at how I can keep up with the big guns out there when I run it with 500 watts an LMR 400 coax. Don't make the height suggestion a hard and fast rule.
Apr 25th 2017, 01:22

wb6lew

Joined: Oct 29th 2013, 12:09
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what is the correct length of a G5RV tuned to 7 MHz
Apr 25th 2017, 15:34

W1VT

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
The G5RV has a moderate SWR of around 4:1 on 40M. With a properly tuned antenna tuner, reasonable efficiency can be obtained when fed with the normal ladder line/coax configuration.

http://www.w5dxp.com/G5RV/G5RV.HTM
Cecil shows how the window line modifies the feedpoint impedance on a Smith Chart.

http://www.karinya.net/g3txq/g5rv/
This page by G3TXQ suggests 2 dB of coaxial line loss on 40M. He also suggests the shorter ZS6BKW as a similar antenna that will cover all of 40M without a tuner.

Zack W1VT
ARRL Senior Lab Engineer

May 11th 2017, 19:52

wb8bhk

Joined: Feb 15th 2017, 01:50
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Total Posts: 0
Used to have a G5RV inverted vee install and worked great on 40/80. Now I am in an RV park for the summer with two large oak trees about 90' apart. Is it ok to install a flat top G5RV and just make the ends dog leg down along the trunk of the tree at each end? Depending on which branch I get the rope through, ends will probably be only about 30' above ground
May 12th 2017, 08:37

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Quote by wb8bhk
Used to have a G5RV inverted vee install and worked great on 40/80. Now I am in an RV park for the summer with two large oak trees about 90' apart. Is it ok to install a flat top G5RV and just make the ends dog leg down along the trunk of the tree at each end? Depending on which branch I get the rope through, ends will probably be only about 30' above ground


That should work OK. Another option, it to build the Compact Multiband dipole I published in the March 2016 QST, that is 43 to 48 ft long. It doesn't do 80, but works on 10 and 20 without a tuner. The G5RV is typically a poor performer on 10M even with a tuner, though the mini version works better on 10M. My antenna is optimized for 10 and 20 meter operation.

Zack W1VT
ARRL Senior Lab Engineer

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