ARRL

Forum Home - Rules - Help - Login - Forgot Password
Members can access, post and reply to the forums below. Before you do, please first read the RULES.

Need a Plan B for a G5RV

Mar 5th 2013, 16:14

K4DUB

Joined: Nov 7th 2007, 14:15
Total Topics: 0
Total Posts: 0
I'm hoping for a workaround (save face) in order to get on the air (HF) ASAP. Here's my problem. I'm trying to convert an 80 meter dipole into a G5RV but the TV twinlead matching section is around two feet (2') short of the 28.5' in the specification.
There aren't any stores in my area that carry twinlead anymore and I don't have any window or ladder-line laying around to save the day.

What should I do to salvage this project in a hurry? Will two (or so) feet less of matching section affect all bands, some bands or ??? Originally, I was going to make this G5RV long enough for 80 meters but now I'm considering going for the 40 meter specifications because things aren't going so well today and this will be a temporary antenna anyway...at least until I can get the materials together to do it right.

Any assistance with this little problem will be most appreciated.
Dub, K4DUB
Mar 5th 2013, 19:52

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

You could add on some homebrew open wire to make up the difference.

According to this calculator

http://www.emclab.cei.uec.ac.jp/xiao/Wire/index.html

half inch spacing and #12 wire will be close to 300 ohms.

To calculate the short feedline length.

Free space length = feedline length/velocity factor=28.5feet/0.82
FSL=34.75 feet
We have 26.5ft/0.82=32.32 ft 34.75-32.32=2.43 feet
2.43*VF=2.43*0.98=2.4 feet of homebrew line

Alternately, you could put it up with the shorter feedline and measure how it does--in practice, computer simulations aren't as accurate as we would like--typical G5RV antennas are affected too much by the surrounding environment--unlike beams on towers which are usually in the clear.

The G5RV is popular because it is a decent compromise on many bands, rather than really good antenna that has to be tuned up. I wouldn't worry to much about trying a compromise version--it may very well meet your needs, or at least meet them as well as any other version you can put up.

73--Zack Lau W1VT
ARRL Senior Lab Engineer
Mar 7th 2013, 09:09

K4DUB

Joined: Nov 7th 2007, 14:15
Total Topics: 0
Total Posts: 0
Thanks, Zack, this is exactly the info I needed. I 'could' leave well enough alone...but that's no fun at all. I'm gonna extend the matching section with my own 300 [or so] ohm twinlead/ladder-line. The calculator you linked for me will come in handy on this little project.
I realize the G5RV is a compromise, except maybe on 20 meters, but it will be an easy way to get my shack back in HF operation after moving to a new location after 20 years at my former QTH. I left my entire antenna farm behind when my house was sold, so now I'm starting from scratch yet again. Thankfully, I have lots of tall pines now in every direction! Even so, I'll still be erecting some verticals as well. So much experimenting to be done...so little time, energy and money. Thanks again, Zack!

Back to Top