I built my full wave loop in a diamond configuration with the feed point at the bottom of the diamond. This makes it horizontally polarized which is what you want for 6 Meters SSB and FT8. Also having it in a diamond configuration greatly simplifies set up since you only need a single point for support. In my case, I use my 40 ft. Spiderbeam pole. One thing I do with the Spiderbeam is I keep the first (thinnest..) section inside the second section as I push up the antenna. I figure this will give added strength at the top support of the antenna so the likelihood of it snapping is less.
Here is a basic diagram of my loop antenna:
Now a full wave loop antenna has an impedance of approximately 100 ohms at resonance. So we need a quarter wave matching section so it will match the 50 ohm coax. For this I use a piece of RG-59 coax, which has an impedance of 75 ohms. This transforms the 100 ohms of the loop down to the desired 50 ohms.
One important thing in calculating the length of this transformer is that we need to know the velocity factor of the coax. Typically for RG-59 there are two types, with different velocity factors. You will need to know what you have. Below are the lengths needed for the two most common velocity factors:
Velocity Factor = 0.66 Length = 38.8 inches.
Velocity Factor = 0.78 Length = 45.9 inches.
Update: It was brought to my attention by a commenter that a good way to tell what the velocity factor is of your RG-59 that you have is to look at the dielectric material between the shield and the center of the coax. If it is white opaque foam then the velocity factor is 0.78 (EG. Belden 8241F). If the dielectric material is semi-transparent, then the velocity factor is 0.66 (Belden 8241). Thanks to Rick, W0YGH for the tip!
Its also important to use the correct type of RG-59 coax. Do not use CATV or cable TV type RG-59 coax. I suggest buying RG-59 from The Original Wireman to insure the proper type of RG-59 coax.
Thanks for sharing Mike- Question on the length of 75 Ohm Coax. Did you change out the connectors for PL-259s? I am thinking this is tomorrows project! Also what length 75 Ohm was used? Thanks again.
Mike
W1SRR
You’re welcome – Glad to see people getting on 6 meters. Concerning the coax, I did not use coax for cable TV. What you need to use is RG-59 coax. If your coax has the typical connectors for CATV it is the wrong coax. In my case, I happened to have it around which did not have connectors on it. So yes, I put my own connectors on.
As far as the length, that depends on the coax velocity factor. This depends on what materials the coax is made of. You can find the velocity factor on the specifications page of the coax you have. Typically it is either 0.66 or 0.78. Once you have determined the velocity factor, the length you need is at the bottom of page 2 of this post.
Hope this helps!
— Mike
This is great, Mike! Just what I’m looking for. I’m curious how you went about tuning this. Do you start with just the loop and adjust for lowest SWR, then add the matching section and repeat? Or some other procedure?
Thanks!
Rick WØYGH
Thanks!
When tuning the antenna, you tune it while the matching section is already installed on the antenna.
For the matching section, I just cut it to length per the table in the article for whatever the velocity factor is for the coax. I then adjusted the length of the loop to get a low SWR.
As far as tuning, I have a MFJ Antenna Analyzer. If you have a SWR bridge that will work too. Hope this helps!
— Mike WB8ERJ
Thanks, Mike!
I found a chart of velocity factors of Belden RG-59. If the dielectric is white foam (Belden 8241F), the VF is 78. If the dielectric is polyethylene (milky white, semi-transparent) (Belden 8241) the VF is 66.
I’ll just cut the matching section to your numbers and go from there.
Rick WØYGH
Thanks Rick, good info!
I will add this to the post
— Mike WB8ERJ