The Handi-tenna (by Jerry, K5OE)
To make the downlink signal of the LEO's more reliable the use of a gain antenna is recommended. Especially the oldest of the very popular FM-birds, AO-27 has a difficult downlink with only around 600mW of power. To provide a good downlink signal a beam antenna must be used.
An intensive search on the web brought me to the very productive satellite related site of Jerry, K5OE. Among many interesting satellite projects you can find his 'Handi-Tenna', an all-day satellite downlink antenna for portable operation. Originally designed for using it on AO-27 the mini beam now could also be used with great succes on UO-14 which Chris, G7UPN brought back into the ham radio community (thank you very much Chris).
The intension of Jerry was to have a gain antenna that is relativly small in size and still is a good performer on that difficult AO-27 downlink. The result was a highly optimized, very portable beam antenna (yagi-uda type), very effective and cheap in building it.
My realization of the handitenna
The idea of a light weighted downlink antenna immediately caught my attention, because the antenna I was using until then was a very heavy 3 ele quad. Not at all fun pointing it a whole pass to the satellite.
To save weight the boom of the antenna consists of isolating plastic tube used for installing powerlines inside the house. This was the only system I could get using a coupling system to connect the tubes. Although it is not very resistant against UV light, you can use it for the original purpose of the handitenna: a portable hand-held beam.
The maximized gain of the yagi-uda array results in a very low impedance of 12,5 Ohm. Using this low impedance the danger of ohmic losses is very high especialy on 70cm, so a direct transformation to 50 Ohm feeding line with a folded dipole was choosen, directly connected with the coax without any balun involved.
The first prototype (still in use)
Depart from the original description on Jerry's site I choosed aluminium welding rods for the parasitic elements to save a little more weight and to avoid the 'soft' cooper. The welding rods do have actually a diameter of 3,2 mm, several tenths of millimeters difference to #10 AWG (=2,59mm) what I just doesn't consider. I used the original lenghts of the elements (for comparison to the original report look at the 'Handitenna' page of K5OE)
The director can be installed in the final position, the reflector first should be fixed removeable to do final SWR tuning after the driven element was trimmed on desired frequency. A sealing band for automobiles etc. did a good job for me.
a parasitic element
The DE consists of 4mm2 copper wire ( = #10 AWG) used for grounding purposes. It is relatively soft, so you can bend the folded dipole easily. I started first with the dimensions Jerry suggested but ended in a different way (see table).
After bending the 5th dipole I had resonance at the desired frequency range of the 70cm satellite band (435 - 438 MHz). The length here is very sensitive, small changes can have big effects (1,6mm is around 1 MHz). A good help is a transmitter with a wide transmitting capability if you end far away from the desired resonance ( I had to expand my transceiver via software to see my resonance at 445 -448 MHz after first attempt).
According to the 'Long yagi workshop' of G3SEK the spacing between the two dipoles can vary in a wide range without serious effects. The recommandation is about 35 - 45 mm , a diameter of 50 mm is too big for 70 cm. I built mine with a spacing of 35mm center-to-center.
The folded dipole is clamped to the coax using stainless steal screws, washers (both sides of the wire) and nuts. The The gap at ther junctions is between 10 and 15mm (using here a 16mm PVC tube). The coax is connected inside the coupling with ring lugs and runs through the tube towards the rear side. There you can connect what ever you want (pre-amp, splitter, rig...)
Fine-tuning the SWR by changing the position of the reflector wasn't necessary. My measurements above gave a 2:1 bandwith between 434 MHz and 439 MHz with a minimum of 1 : 1.3 at 436.8 MHz.
It is a real pleasure to work with that nice little piece of antenna. On UO-14 the signals are great. Even inside the house you can almost ever find a direction where the signalstrength is useable. Listening AO-27 is more difficult due to the lower signals but you can still use it with success avoiding the big antennas giving you horizont to horizont coverage. On SO-35, the signals were also great when he was switched to Mode J (2m up/70cm down). Unfortunately SO-35 no longer is in operation.
Three weeks ago I stayed in a mediaeval town for 2 passes of UO-14 with very unfavourable circumstances for satellite reception. Small streets, narrow lanes obstruct the view to horizont. But I still was able to follow UO-14 almost the whole pass profiting by the different reflection at railings, a lorry and what ever was involved !
The small size, the extreme gain and the light weight make this antenna a real winner in portable operation.
Also in FM simplex you can impress your qso partner telling him you're just running 400mW from your HT.....
Graphical radiation patterns
Other UsersSee the work of XE3RN, who built the original version of Jerry en katalan
Here is an extended version (4ele) de KG4IHJ