2007-01-14
VK2ZOI Flower-Pot Antenna
It had been about 10 years since I used 2 metres actively. I figured it was time to get back into the band. I saw the VK2ZOI "Flower-Pot Antenna" mentioned on the ARNSW website, so I wrote to John Bishop and asked for his documentation on the antenna. John replied quickly with three detailed articles about his antenna and extracted a promise from me to document my experiments with his designs.
My first problem was assessing the antenna's match to the coax feed at 2 metres. I had to build a new Z-bridge to enable measurement of the reflection coefficient. I intended to use this device largely qualitatively (ie, trim for peak return-loss), but it is perfectly capable of quantitative measurements with some care in its use.
I made my first attempt with a "naked" version of the vanilla 2 metre 1/2 wave antenna, as I did not have the 25 mm conduit called for in the original design. I cut it long and pruned it down. With 500 mW from the FT-415 HT on low I got it to the point where less than 10 mW was bounced back - I considered that pretty good for the very primitive lash-up I had and used that version of the antenna with the HT for a few days.
My initial efforts with the old FT-415 HT were frustrating, not only was its battery pretty much cactus, but its front-end suffered severe intermod problems from a local paging transmitter just above 148.0 MHz. (I don't remember it being this bad? Guess it just gets worse every year.) With its rubber ducky, intermod was barely acceptable, with the VK2ZOI antenna it was next to useless. I did however get into a few more repeaters and have a bit of fun.
I decided to throw some money at the problem. I'm almost ashamed to admit it, but I purchased a FT-2800M mobile 65 Watt FM transceiver. I'm not an appliance operator - I sware! :) Considering it cost around the $300 mark, I doubt I could have built something as functional for that - just in parts, let alone the several man-weeks of my work required to design and build it.
The positive side is that I now have a QRO-capable radio, and that's always useful. I also now have a radio with a sub-tone encoder. Sub-tones are more common now than when I last used 2 metres, to avoid the ever worsening intermod garbage breaking into repeaters I guess?
In the meantime I had purchased some inch conduit. Unfortunately the local Bunnings only had the "orange" stuff in stock. John's article specifies "grey" conduit, and suggests orange will have different electrical properties, altering the design somewhat. So, once again, I cut the antenna long and pruned it to a match.
Not quite as good a match, perhaps I need to extend the braid-side a little? Fortunately this is pretty easy, you just uncoil the choke and push a bit more coax up inside the tube (which should be cut long for this kind of adjustment), then rewind the coil.
My first QSO with the "orange" antenna and the new FT-2800M was with John VK2VSS using the 146.850 repeater down the coast. The "local" 147.025 repeater absolutely screams in. My initial target 147.0 I can now break the squelsh of, but I am yet to have a QSO on the repeater to assess how well I am making the trip. I hear the repeater at about S2-S3 - very noisy. This is probably more a problem of my QTH and the placement of the antenna. In the distant past, when I used a commercial (big Diamond co-linear) antenna, up much higher, I could access this repeater with a small power amplifier on the HT, but it was never "strong". IMO the current antenna is doing pretty well, but it could do better.
More experiments have been put on hold while I build some more test equipment. In particular, I want to measure the self-resonance of the choked feed system. I tried a lash-up on the bench with a signal generator, resistor and diode detector, it was a little too unstable to make real measurements with. I need a step attenuator and perhaps a VHF-capable dip meter (both of which I continue to promise myself I'll build - next week...). An interesting way to measure it is to place the coil in series with a noise source and plug it into my VHF/UHF spectrum analyser. This gives a clear picture of the "trap" function at resonance, but again my primitive test lash-up means I pull the tuning whenever I approach the coil with my hand. The trap is quite low Q however, so this is probably not of huge consequence.
Still, I want to make real, repeatable measurements if I am to do serious work with the design. I'll also have to find a more open-air location for my tests, my balcony isn't really appropriate for a VHF antenna test-range.
I've since purchased "grey" conduit for comparison tests, and a few niceties, like an end-cap, some stainless hose-clamps to mount the antenna, etc. In time I'll work my way through the logical course of experiments. I also want to build the 6 metre version. 6 metres is a band I really want to build a radio for - maybe in a few months.
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