Mobile Antennas

Posted on Wednesday, March 20th, 2013 at 7:15 am in



Recently had a nice chat with somebody about my experiences with mobile antennas and here is what I said in my e-mail:
What radio are you using? I used to have a Yaesu FT-707 that I pulled out of the mothballs but the VFO wasn’t stable enough any more. Then I bought a second hand FT-747 which I’m extremely happy with and I will probably keep it for home use. Then recently I bought a Yaesu FT-857D from OM Sam Maree in Cape Town and I’m also happy with it except for a few niggling things. The FT-857D is an excellent mobile rig, and the combination of the FT-857D plus the FC-40 automatic tuner is a killer combo. You have NO idea how well they work together until you actually use them. Awesome stuff.

The FC-40 tuner will tune just about any random length of wire to the transmitter and was built specifically for that purpose. When I go mobile I just use a kettie to shoot a thin piece of fishing line into a tree and I go. My wire length is usually 10 meters or so of thin copper wire. You should try that tuner out sometime.

I also tried a vertical without traps but I get variable results, and I was going to try a vertical with traps next. My experience with home based stuff is that verticals are a last resort, except if you can get a good early model bandspanner or similar. The good verticals tend to be monobanders too, unless you put your hand deep into your pocket for a screwdriver. By which time I’ve lost interest.

My best antenna, by sheer luck, and by far, turned out to be a G5RV. I eventually purchased a “G5RV Junior” from Sam (Radio Accessories in Woodmead) which has become my reference against which I check all my other experiments. I’ve done dipoles with and without traps, a “Terminated Folded Dipole” from Buxcomm, end-fed wires, the lot. Each has some advantage and some disadvantage. The disadvantage of the G5RV, in spite of its excellent usability, is that you have to use it with a tuner as the impedance varies a bit. I actually don’t mind this, as I bought a second-hand Yaesu FC-757 ATU which is semi-automatic but works very well. I have never felt that I lost a signal because of the tuner – but the purists will argue till dawn. You can actually make a lightweight inverted-V G5RV that works extremely well with simple wires, if you can find pink 300 ohm TV wire. If I can find a few meters I will pay with solid gold. It outperforms any dipole under similar conditions, and I think it has to do with the feedline matching.

I also have an old Army wire antenna complete with traps and balun for 80/40/20 which makes an easy V for field days but I don’t seem to use it a lot nowadays. I don’t have the vertical pole though … pity!

I must admit my biggest problem – for mobile use – has always been power. Dragging a 12v/20A supply around is just a pain in the back. By the time you’ve dragged your power supply out from the boot of your car to where you put your antenna, it becomes attractive to rather pull your car up to where the antenna is. In that case, even a well-tuned vertical monobander sounds good. I know I’m lazy … but the radio also gets bumps and scratches when you haul it in and out, so a somewhat fixed installation feeding directly from the car battery with short leads (less fading on high power signals) is a very good compromise.

Well, thats my opinion for the day. Have fun whatever you do and please keep in touch – I would like to make a contact on the air sometime. Chat soon.