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Well, if you have been following my other posts you will know that I live next to a monster of a power line servitude, less than 40 meters away from my shack. And 40 meters high. On my favourite HF band the noise levels almost hurt your ears. Something had to be done.

I tried a few things such as ferrite beads and a folded dipole but no real improvement. Scanning the internet the magnetic loop (small transmitting loop) antenna often came back as a lower-noise antenna, and after visiting Eddie (previous article) I decided to try my hand at building an experimental prototype. There was a lot of information available on the internet and I will add my plans and drawings to the bottom of the article as downloads. The achilles heel of these antennas is the high voltage capacitor, and after searching through two flea markets unsuccessfully I decided to make my own.

Essentially I wanted a copper water pipe loop that can be resonated around 7.060 MHz, as that would allow me to compare the results with my other radios and antennas. So I purchased a length of 28mm copper pipe and 8x 45 degree elbows. I also purchased a length of 50mm white PVC with which to hold the whole construction in an upright position. Initially I cut 7 pieces of 620mm each and soldered them together in a loose octagon with what seems to be a diameter in the 1550mm region. The 8th leg was to be decided later, and two loose pieces of around 200mm and 400mm respectively was soldered in temporily.

I need to say, right up front, that I am not a purist. While I will do strange and wonderful things to reach the optimum, my favourite motto is “MOL” (More Or Less). Get the basics right and the rest will take care of itself. So the few compromises I was willing to make are:

• Not a round shape, but an octagon (8 sides)
• A trombone capacitor
• Feeding scheme to be decided later

Initially just to see if the loop will resonate at all I tried feeding it with a piece of circular coax in a faraday loop configuration but that didn’t really give strong results, but I think it was my own fault because I was standing too close to the loop. As soon as I soldered in a tab for a gamma match, things changed drastically.

First I tested with an old valve receiver spaced-plate capacitor (no power transmitting yet) and my antenna tester to see MOL (more or less) what frequency I’m getting. Wow, it was all over the show and I couldn’t get any useful readings. A cup of coffee and lots of staring into the distance later and I realised that the adjustments I’m making on the capacitor were WAY too much. In fact, lesson 1: To tune across the ENTIRE 40m band (100KHz) you only need from about 118pF to 121pF !! (yes, around 3Pf). Lesson learned: Make very small changes. Reference: LOOP CALCULATOR

After that it was plain sailing to the first QSO, I didn’t even bother to make proper solder connections for the gamma match and literally clamped a piece of 1.5mm electrical earth cable to where it resonated around a SWR of 1.5 and I fired up my set. Bingo, very nice contact with ZS6TY (Johan) and the concept is proven. At this stage, remember, I was using an in-line monoband trombone capacitor, and 1mm linear adjustment would give me a 25kHz shift in band so it was very sensitive, but it worked. In comparison to my G5RV when the noise was around S9 on the dipole, the loop would be S6 and the audio quality more or less the same. Definitely an improvement.

To see how I made the capacitor download the diagram at the bottom of the article.

Now for version 2 I wanted to make a remote-control capacitor. I don’t want to spend money on making it automatic yet, but I wanted to at least be able to change the capacitance from inside the shack. So out comes the soldering kit, out comes the two short pipes and they are replaced by two 300mm pieces. The old trombone capacitor is now turned through 90 degrees and the inside element is soldered to the one 300mm piece with a T-piece (look at the diagrams below). The outer element gets a 5mm brass nut soldered onto an end-cap with a 500mm long 5mm threaded link. A plastic coke bottle top becomes my adjuster on top and voila! the new antenna goes up.

In measuring I find the loop has a natural resonance at 24MHz (no additional capacitance inserted). As soon as the capacitor gets connected it can be tuned to 7.060MHz very easily, and the gamma match can be set equally easily. Unfortunately I offset the feed point a little to make room for the supporting pole, and I will have to go back and clean up the construction.

The next few weeks will be spent doing fine tuning and documenting the antenna. At the moment I’m pointing slightly west of north to get past the power pylon and hopefully catch some north-west ZS amateurs, but I was happily chatting to people north-east of me as well so I’m not too sure how deep the side lobe nulls are, yet. As it stands, it is a cloud warmer because I have no chance to get out horizontally or even at 60 degrees elevation.

Please write me for details if you need them?

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.

73s

As you might have gathered by now, I restarted my ham hobby after being absent for many years. The absence also meant that my antennas were either bent out of shape, or missing some critical parts.

Panic stricken I rushed off to my local purveyor, Sam Ford ZS6___ of Radio Accesories in Woodmead waving my credit card at anything that looks like it could receive radio waves. After calming my nerves, Sam pointed out that he has a MFJ-1778M in stock. What? I asked. So he says: Single dipole, multi-band, no traps. OK, he has my attention.

It turns out to be the best advice ever. The MFJ-1778M JUNIOR G5RV is supposed to tune up on 10-40MHz, can handle 1500 watts (OK, I only have 100 watt at the moment), and is just under 16 meters wide. Now thats impressive, and will fit my chimney mounting strategy. It took me a full hour to hoist this lovely piece of equipment onto my tower AND make a 15 meter RG58 cable, and before tea time on Saturday I keyed up the tuner.

Initially I was very disappointed, it tuned up on 40m and 10m but not on 20m. Reception was very good, although remember I am in a noisy environment. Then the following weekend and after discussing with Sam again, I decided to lower the antenna and create more of a “Cloud Warmer”. Initially I came down a meter, but after some more experiments I ended up with a nearly horizontal antenna no more than 3 meters above ground level. It may be a cloud warmer but it tunes up instantly on any band between 40m and 10m (including the WARCs) and so far it has become my all-time favourite antenna. It is the standard against which I judge all my other experiments.

Although the picture is very skew and not a good rendition of the antenna, I built myself a “test” version of the Balanced Termination Folded Dipole (BTFD) as shown on his website by Buck Rogers Sr K4ABT of BUXCOMM.

Now before we carry on I need to acknowledge my friend and colleague Bruce Rodrigues who used to say: “There is nothing more permanent than a temporary fix”. This antenna might go in the same direction, I like what I hear.

Firstly I wanted to find out for myself if the folded dipole, terminated in the middle with a 450 ohm resistor, would be more quiet and still receive low level signals. So I made a rough hand sketch of two side loops with 21 meters of wire each side. I didn’t have proper antenna wire so I used electric fence wire – lots of that around. It is a multi-strand wire, silvery-looking, and it is very stiff so it must be steel or something. I bought a 10mm dowel, cut it into 220mm pieces and drilled 2mm holes near the ends for he wire to pass through. I purchased a random 25 watt 470 ohm resistor (I have no idea if it has inductance or what) and with electric fence crimp collars wired up the works in less than an hour. I’ll post some photos later. The balun is a problem as I don’t have a 9:1 balun available, so I used my standard 4:1 balun and I’m expecting some degradation because of that. I strung it from the highest point on the chimney pole to a roof eave in a slightly E-W direction.

The photo shows the middle area of the antenna with the resistor on top and the balun below.

Comparing it to my MFJ Junior G5RV on-air (below the BTFD in the picture) it was giving very similar signal reports, and it was about 2 S-points quieter. This is enough to do some more scientific measurements and make some refinements, like replacing the balun. Initial impressions were very favourable and I’ll keep you posted.