VHF End-Fed Half-Wave J-Pole
Antenna for High Altitude Balloon Use

May - 2006

This is the impedance matching transmission line portion of the 2nd APRS antenna built and used for SABLE-1 & SABLE-2.
The feed line enters at the left end and the first few inches of the half-wave radiating element can be seen at the right.

I needed an antenna for SABLE-1 and was tempted to use a small helical rubber duck antenna that could be installed on the tracker and simply placed into the payload box with it. But a rubber duck is a terrible antenna that's only designed for short-range hand-held use. It's also a 1/4 wave antenna that requires a ground plain or some type of counterpoise to operate properly and I wanted something better that didn't require a ground plain, like the half-wave center-fed folded dipole antennas we've been using. I doubt there's anything easier to build that would perform as well as the folded dipole antennas, but a half-wave end-fed antenna that can be simply hung from the payload box would be much easier to use so I once again began searching the web for antenna information and proceeded to try and build one.

August - 2007

This page WAS and I HAD planned to finish writing about the two end-fed antennas that were built and used, but what you see is all there will ever be as I forgot to take many photos of their construction, have lost all the scrapes of paper with measurements and other info, and have too many other projects & too little time.

These are J-Pole type antennas with 300 ohm TV twin-lead feed-line for the matching section and piano wire for the 1/2 wave dipole. They work excellent, are fairly easy to build & fairly light weight, but nearby objects affect them much more then the Folded Dipole Antennas, tuning requires a lot of trial & error and time (see below) and they are heavier then I would have liked for balloon use.

I tried a number of different coils, tapped coils and transformer designs for matching 50 ohm feed line to the high impedance at the end of a half-wave dipole, but they were all extremely sensitive to anything nearby and very hard to adjust for a decent match.

The matching section was built similar to the Folded Dipole Antenna, with balsa wood strips and epoxy resin for strength and protection, and the dipole was made from 1/16" dia. piano wire. Hobby store piano wire is normally only 36" long, but the dipole is a bit longer so a 36" length was extended with a short piece near the top of the matching section using a small length of brass tubing over the joint and silver solder for strength. SABLE-1 proved this joint wasn't strong enough so a 2nd piece of brass tubing, of the next larger size, was used over the first. More important was that the missing piece of 1/16" dia. wire was replaced with 1/32" dia. wire which I didn't have to use when building these antennas.

A perfect 1:1 match was able to be achieved at 144.390 MHz

VSWR was

VSWR was

< 1.2 from 143.0 to 145.8 MHz
< 1.5 from 142.3 to 147.0 MHz

= 2.2 with the antenna laying flat on the earth.

Antenna #1 Weight:
Antenna #2 Weight:
 82 grams (2.9 oz)
 80 grams (2.8 oz)
Length = 55"

 A few photos were taken while testing the finished antennas and the one pictured above was the 1st one built and prototype for the one seen at the top of the page. The matching section was sealed & protected using heat shrink tubing and doesn't look that great, but the antenna works very well and was used for SABLE-3 whose 300mW APRS signal was heard over 600 km away.





Left - The 2nd antenna suspended several wave lengths above the ground during a test.

The mess at the end of the wooden support is a GPS, tracker and battery in a zip-lock plastic bag for protection from occasional rain showers while finding how long the battery would last.

A similar arrangement was used to tune antennas, with the feed-line from the antenna analyser run along the wooden support and down to the antenna to keep it well away and from affecting the antenna.

The 2nd antenna looked better with the matching section placed in a thin walled tube made using several layers of light weight fiberglass cloth and epoxy resin. It was used for SABLE 1 & 2, but was lost when SABLE-2 was not found. I would make another one like it, but too much time & effort is required to obtain a good match at the right frequency with tuning affected the way it is by enclosing and sealing the matching section. It becomes a lot of trial & error, with guessing how much tuning will be affected, tuning to what's figured may be correct, enclosing & sealing the matching section, checking the match and then, if not correct, unsealing everything and starting over again until correct & the best match is at the right frequency.

If I were to build something similar, I would use 1/32" piano wire for less weight. It would also put little stress on the brass tubing splice, but I would look for it in longer lengths to simply eliminate the splice. Also to eliminate the 300 ohm feed-line by forming a continuous piece into a 'J' for a more typical J-Pole design. Finding a way to leaving the adjustment point 'open' would also make tuning much more simple and easy.

Right - Two nylon cords were embedded with the feed-line in the epoxy used to seal the end of the matching section and were used to hang the antenna under the capsule. One could simply use the feed-line for this, of course, but would likely cause it to become damaged before long.

Left - A safety bead for the dipole end is easy to make with a blob of 5 minute epoxy. Slowly rotating the wire while it's kept basically horizontal until the epoxy cures will provide a nice symmetrical bead and filing a few notches in the wire first will keep the bead from being pulled off.


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