The Back-Pack-a-Tenna

Summer is right around the corner and the beauty of the Virginia landscape is in full swing. The Shenandoah Mountains are a short skip from Fauquier County and are begging to be climbed, providing an excellent platform for amateur radio propagation. Summit on the Air[1] activations are a fun way of combining amateur radio with the great outdoors and have peaked my interest for sometime. My original investment in an Elecraft KX3[2], while suboptimal for home use, was intentional to support these kinds of activities. Since purchasing the KX3, I have been slowly accruing the necessities to enable a SOTA activation adventure, including antennas, batteries and more. While totally superfluous, I have constructed yet another accessory to enable effective APRS[3] beacons and constant VHF/UHF communication while hiking to and from the summit. Behold the Back-Pack-a-Tenna.

The Back-Pack-a-Tenna is a vertical dipole attached to the exterior of my hiking backpack.  The primary goal is to enable better signal radiation over a traditional HandieTalkie with inferior rubber ducky while on the trail and at the summit.  The secondary goal is being a proper Fauquier Radio Geek!

My hiking backpack is a military style bag[4] with MOLLE webbing on the exterior for attaching all manner of accessories.  The webbing holds secure a radio pouch[5] and a ½” CPVC tube for the antenna mast. A short piece of RG-8X cable with male SMA connectors[6] attaches to the radio and feeds through the CPVC to the tip of the mast.  The CPVC mast is held in place by a Tee connector between levels of MOLLE webbing preventing the mast from slipping down.  A speaker hand mic wraps around the mast and attaches to the left shoulder strap.

The primary element of the antenna is a 2m Signal Stick[7].  I chose the Signal Stick over other similar aftermarket ¼ wave antennas due to its superior flexibility.   The Signal Stick should easily handle the occasional branch or other obstacle while on the trail.   An 19.5” tiger tail of 18 gauge antenna wire makes up the second half of the dipole.  We learned from our experimenter extraordinaire, John KX4O[8], that using a tiger tail with a ¼ wave aftermarket antenna is superior to using the ¼ wave antenna alone.  The Signal Stick and tiger tail are attached to the feedline with a female to female SMA bulkhead connector[9].  The bulkhead connector provides the base plate and fastener for attaching the tiger tail.  The final element is a Mix 31 Clamp-On ferrite[10] near the SMA bulkhead connector to prevent common mode currents on the feedline.  A Mix 31 ferrite is ideal for 2m transmission with a peak impedance near 150MHz.  If you prefer 70cm operation then a Mix 61 ferrite would be a better option as it shifts the peak impedance frequency toward 440MHz. The antenna assembly then sits semi-snuggly into a ½” to ¾” CPVC fitting at the top of the mast.  The ¾” CPVC fitting internal diameter is conveniently the perfect size for the ferrite.

The physical construction of the Back-Pack-a-Tenna proved better than I had anticipated.  The antenna mast stays mostly vertical and the speaker hand mic is located perfectly with the backpack slung over my shoulders.  But how does the antenna perform electrically?  Without the sophisticated test equipment available to John KX4O, I fell back to my trusty antenna analyzer. With the Back-Pack-a-Tenna on my back and in the middle of my backyard, I was able to capture an SWR plot across the 2m band.  I observed an SWR minimum of 1.03 at 145.47MHz and stayed below an SWR of 1.5 over the full 2m band.  I may be able to shift the center frequency to the right by shortening the tiger tail but this experiment will have to wait for another day.  The SWR sweep was followed up by quick functional test to the FARA repeater and all was well.


The only thing left inhibiting my Summit on the Air activation are excuses, which I seem to have in abundance.  In the near future I will be heading up to Skyline drive and the beautiful Shenandoah National Park[11] for an attempted SOTA activation.  I’ll be working 20m and 2m and will need four contacts for a successful activation.  Please keep an ear out on the FARA repeater as I’ll be chatting away on my way to the top!

[6] – Part Number 721405423101
[9] – Part Number 721405424689
[10] – Part Number 623-0431164281