Doing some internet research I found references for dual band 450 ohm ladder line or twin lead J-Poles that were simply J-Poles that worked off the 2 m 3rd harmonic for 70 cm, but with less than desirable radiation patterns and performance, which was also my experience. Taking a que from Edison Fong, WB6INQ, I decided to adapt his DBJ-1 Dual Band J-Pole seen in the ARRL Feburary 2003 QST issue from 300 ohm twin lead to 450 ohm ladder line, which is more readily available and familiar to amateur radio operators these days.
The DBJ-1 called for mounting the antenna inside of 3/4" 200 psi PVC pipe which has a thinner wall thickness than Schedule 40 PVC. While I've had good results mounting twin lead antennas on the outside of Schedule 40 PVC pipe, mounting them inside of it has been a problem, most likely due to the RF attenuation and interaction with the thicker material thus leading to SWR issues.
My first attempt was to build the entire length of antenna with 300 ohm twin lead and then trim for SWR. There are a couple of types of 300 ohm twin lead available, thickly insulated outdoor stuff, and flimsy indoor TV stuff. I tried both types and had poor results, testing both outside and inside of Schedule 40 PVC pipe.
I then decided the best way to proceed might be to build each segment of the antenna with 450 ohm ladder line, which I find easier to work with, and tune each segment for low SWR before adding the next section, and then testing it inside of 3/4" 200 psi PVC pipe.
When testing the antenna I wanted to keep it away from objects that might interact with it so I used a 10' stick of PVC pipe. Since the actual driven element side of the antenna is 53 5/8" long, the PVC pipe could be shortened by about 43" and the same amount of coax removed. This would minimize unnecessary line loss from the extra coax housed in the pipe and be less prone to swaying in the wind. Due to the thinner wall thickness of 200 psi vs. Schedule 40 PVC it's probably a good idea to keep any unsupported sections of it to around 5' in length.
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| Antenna is housed in a 10' length of 3/4" 200 psi PVC pipe. |
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| A stub of RG-8X coax shorted at the top acts to seperate the VHF/UHF segments of the antenna. |
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| Antenna length with 450 ohm twin lead is 53 5/8" vs 50" of 300 ohm twin lead for Ed Fong's DBJ-1 |
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| RG-8X coax connected to the SO-239 connector mounted inside of a 1 1/4" PVC plug. |
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| 1 1/4" PVC plug to 1 1/4" coupling, and 1 1/4" x 3/4" bushing to antenna housing. Antenna is fed with LMR-400 coax. A small section of 1 1/4" PVC pipe could be added to house a few turns of coax if needed for a RF choke. |
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Channel Master CM-3080 chimney antenna mount.
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After multiple iterations of trimming and taking SWR readings before adding each antenna segment, I decided the 2m SWR was pretty good and that I could live with the SWR at 70 cm. At 70 cm it's useable from 435 MHz to 449 MHz which is the portion of the band I'm interested in.
The final installation has the 10' PVC antenna assembly attached to a 10' stick of 1" EMT conduit mounted to the chimney. The conduit adds support so that the PVC segment doesn't sway under wind load. The top of the antenna is about 35' above ground level. The PVC and EMT segments were painted a flat primer gray to make them a bit less noticeable.
Results have been good thus far with one station on 2 meters simplex reporting S9 received signal strength with only 5 watts output to the antenna from 23 miles away and another simplex station on 70 cm reporting 10 over S9 received signal strength from 17 miles away with 50 watts output to the antenna.
- 73's!
WB5WEC
