Monday, April 21, 2014

Half Size G5RV

The first antenna I ever bought after gaining my licence was a G5RV. I think it cost me around £30 and it used to hang in my garden, squeezed in some tiny area at the back of my house! But I couldn't complain I got a signal out and had contacts all over Europe and the east coast of the States. It suddenly dawned on me last week that I had never actually built one and it was about time I put that right. 

If you trawl the websites you’ll find plenty of examples and I decided upon a half size build, this purely because I had my old Mystery antenna sitting in the shed doing nothing and I could cannibalise that and use the wire for the half size G5RV. 


The W5GI Mystery antenna is similar to a G5RV but W5GI (now SK) used coax stubs to give what he called “a coaxial colinear array on 20m”. It consists of a half wave length (at 20m) of 300 Ohm ribbon that feeds a dipole centre. I made one many moons ago as an experiment (see previous posts) and it worked very well. 

The measurements for a half size are listed below and with having the mystery antenna already made up it was a quick case of re measuring and then trimming wire where necessary. I noticed some measurement do vary especially the matching section, (look at the picture which states 17' and the table that lists 15.6'), which is correct - take your pick?

So by the end of an afternoon I had the antenna made up and ready for hanging in my silver birch at approximately 25ft. As a change I decided on an inverted vee and after a quick double check of all connections and making sure the angles were good I connected up via manual ATU to the FT 1000 MP. 

TABLE OF FULL-SIZE, DOUBLE-SIZE and HALF-SIZE :
Band Coverage
3.5 - 28 MHz
1.8 - 28 MHz
7 - 28 MHz
Length of Antenna
102 ft ( 31.1 m )
204 ft ( 62.2 m )
51 ft ( 15.55 m )
Matching section :



- Open wire
33.7 ft ( 10.28 m )
67.5 ft ( 20.56 m )
16.9 ft ( 5.14 m )
- Ladder line
31.3 ft ( 9.54 m )
62.6 ft ( 19.08 m )
15.6 ft ( 4.77 m )
- "TV" twin lead
28.5 ft ( 8.69 m )
57 ft ( 17.38 m )
14.3 ft ( 4.35 m )

Normally I would put any home brew antenna through the MFJ 259 for checking, but the SWR readings on the G5RV would not be 1:1 and would certainly have me scratching my head trying to get all the bands to an acceptable level. In the case of a G5RV you use the tuner to do most of the work.

So initial results are I have to admit, surprisingly good! 
I manage a couple of QSO’s on 20 meters and on 30. Interestingly 40 meters proved more difficult to tune up, but eventually I got there with some gentle adjusting. I also tried all the bands with the auto tuner on the FT, all was OK except 40. I then brought out the MFJ 259 and SWR on 40 was way up compared to other bands. 

Checking on the web I looked for any mistakes I may have made but the only conclusion I could come to was it maybe an issue with the height as 25ft is not that high and at 40 meters virtually the whole of the antenna is used to radiate. 

Also as a precaution I added a choke balun at the base of the 300 ohm part of the antenna. Of course this where the 300 ohm feeder is attached to the 50 ohm coax (or to be absolutely correct the 75 ohm). You'd think you'd have a 4:1 balun, but then I've never really been an antenna theorist, I just build them!

Further reading from the web I came across the W2DU balun which looked to suit my purpose nicely. It states, "The W2DU bead balun consists of 50 FB-73-2401 ferrite beads over a length of 1ft coax. 

W2DU

Sadly not having the correct beads I added what spare ferrite rings I had to the coax, which for the moment hopefully may have done the trick but I will find some proper ferrite beads at a later date. Unfortunately the W2DU balun looks to be only available in the US and although fairly cheap the shipping costs to the UK are not! So I will either have to replicate with a home brew or find some other 1:1 Balun. 

A mish mash of ferrite rings but it seems to work!

Now manually adjusting the tuner for 40 meters is a little easier but in theory it should be the easiest band to tune up, so I think something is still not right, (probably the balun) but for the moment I'm just happy to get out and  have fun with my new antenna!

As a Further update I have been reading up on baluns for G5RV's and what a can of worms you can open with the statement " to have or to have not"! 

It looks like my home made necklace of ferrite cores will not do the trick, so today After reading G3TXQ's interesting article on the G5RV I have taken his advice and bought a simple 1:1 Guanella balun. 



The Guanella balun fits nicely in to an old plastic screw container, it's now a matter of just connecting up to the G5RV and see if the performance will improve.





Steve Hunt, G3TXQ is a bit of a guru to me, he has a wonderful website all about Amateur radio with a lot of very useful information on antennas and antenna theory, well worth a look especially if you are in to HexBeams or Cobweb antennas.






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