CB silver rod as multiband HF antenna?

Yesterday i visited a new ham (passed exam last tuesday) to deliver a power supply. He allready had bought some second hand antennas, one of which was a silver rod (or GPA or whatever they are called) He had removed the matching spool and replaced that with a 1:9 Unun. I saw the readout of the SWR on his nanoVNA which was about 1:4 or lower from 7Mhz all the way up to 90Mhz. So this antenna with an ATU would be very usefull as an HF multiband. I saw it, but couldn't believe it. What am i missing? SWR is not the holy grail, but what would the problems of this antenna be?

73
Karel
PS: If that works, i still have such an antenna laying around 

From an internet search, a siver rod is 16'5". This falls very close to 16' which is a length to avoid for a random wire to match 50 to 75 ohms to the rig with a 9:1 transformer for HF. However, at such short lengths other factors may come into play; feeder length for example. The only way to see what's going on is to test at the point where the coax goes into the rig with an analyser. It's an interesting topic though and should be explored further.

Deb

From what has been described it sounds he's made something like a GPA-80 or similar no radial verticals. ATU will probably bring into line on many bands and I'm sure it will work, but I bet more satisfactorily on some bands more than others. Coax might be acting as a counterpoise. Fair play to him if he's made a cheap option to get him on multiple bands.

All SWR shows you is there's an impedance match, which has nothing whatsoever to do with radiation efficient.  Remember, a good dummy load gives a 1:1 SWR, but it's not a very good antenna.

There are many published lengths for antennas that give an acceptable SWR with a 9:1 unun, and I suspect electrically he just happens to have hit one of those. 

Nothing wrong with a compromise antenna either, if it's all you have.  Most important is getting on air, and learning as you do so, if he's a new Ham.

I'm going to explore further. Had an old silver rod laying around, so did my son. Did some restauration to it, made one from two, cleanup, changing the bolts and clamps with stainless versions. Removed the spool fom the bottom and replaced the SO239 with 2 wire connectors.

Furthermore i modelled the new setup in CocoaNEC. The outcome of this evaluation was that:
- the use of a 1:4 unun gave a lower SWR then an 1:9 unun. On 40m 10.1, lower on 20 and 10m (the only bands i'm licensed for)
- i usually use the Spiderbeam pole or a fiberglass surf mast. Taking into account an aluminum tube as antenna support improved the results
- modelling the coax outside as a couterpoise also gives improvement.

To do:
- put up the antenna with an aluminum tupe as support, using a 1:4 unun and RF choke.
 (done, antenna length 5.5m)
- analyse the results with my nanoVNA.
(done and still todo with changing setups)
- See what difference the RF choke makes
(without RF choke SWR on 40-20-10 was lower, but when connecting the ATU and radio it didn't get a reasonable tuned SWR, especially on 40, with RF choke i got good results (no beeps) on 20 and 10 and 4 beeps on 40 which means SWR< 3.0)
- See what influence varying the bottom height of the alu tube has (between 0 and 2.60m).
(Still to do)
- Analyse the influence of the length of the alu tube (3 or 6m)
(Still todo, now i have mounted the antenna on a 2m alu tube)
- Does lengthening the antenna improve results? I could add 1m(as seen in the pic), which would make the length the same as the commercially available GPA-80HF.
(Still todo)
Also:
Back to modelling, varying the height and length of the antenna and varying the bottom height of the alu tube, to predict improvements




To be continued.

73
Karel