Last weeks project with the 6m Delta loop went well. During tuning I could only get the swr down to between 1.3 and 1.4. While constructing the Delta Loop, I did not recalculate the length of the 75 ohm cable used for the matching and suspect this is the issue. This week we will build a new matching section using measurements calculated with the velocity factor of the cable.
Over the week I have noticed the 1/2″ pvc starting to bow. Since I had plenty of 1 1/4″ pvc on hand, I cut a full section for the boom. Also While digging in the shop I found yellow 14 awg THHN. In an attempt to make the antenna more visible in pictures, it was used to replace the radiating element.
Length of the loop has not changed. The new wire was cut at 20′ 2″. Extra is so you don’t have to add wire during tuning. The main change to the setup is the matching section. After digging around, I found 2 good candidates for the 75 ohm coax.
RG-6 Quad Shield Belden 5781
Construction of this matching section is the same as the original. The Velocity Factor for this cable is 84%. The formula to find the length of the matching section is “75 ohm cable length =246 * velocity factor / frequency”. The result will be in feet. To calculate meters use, “75 ohm cable length =75 * velocity factor / frequency”. The desired frequency in my case is 50.313 Mhz. This works out to 4.107′ or 49.28″.
After attaching to the antenna, it was analyzed. The resonant frequency was low, but there was a wide area at 1.2 swr. This was very promising. No tuning was done with this matching section. When compared to the next type of cable, they were resonant at different frequencies. The RG-6 was slightly higher.
RG-11 Tri Shield Belden 9764
The RG-11 is thicker and stiffer than RG-6. This particular cable has a water repellant or “Icky Pick” on the inside. This stuff is great when making antennas but can be a bit messy. I have found the solvent “Goof Off” works well to remove the Icky Pick from were you will make connections.
The Velocity Factor for this cable is close to the RG-6 cable at, 83%. The length works out to be 4.058′ or 48.67″. Since the bend radius of this cable is so much bigger than the RG-6, its coil is much bigger and does not have a form. With a little care and some zip ties you can make a nice loop.
With no tuning, the antenna had 1.3 SWR. Since the loop was cut a little long, removing a little wire put the antenna were I want to use it.
The antenna can be shortened a little if desired. Since radio testing will be done with FT8, This range will work well. The RG-11 matching section does not have good strain relief. Screwing the terminal strips to a small piece of pvc may be a good solution. As this project is tested and progress’s there will be future posts.
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