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

Had an error in my Cocoanec model. The difference in diameter made the aluminum pole not taking part in the radiation. Had to draw a small connection between the feedpoint and the alu pole. Then it was no longer necessary to take the coax into account( so simulating a working RF choke ). Tomorrow i'm going to get me a 6m alu mast to put the antenna on. According to my modelling it will be kind of a vertical dipole. Today the 1:9 balun came in. Connecting it and using the antenna on the 2m alu pole also made the 40m tuneable. BTW, made the antenna 5.35m (half wavelength on 20m).

I'm back to this setup after a short experiment with an ATAS-120. The 6m alu pole is in, also ordering a 1:9 unun instead of the 1:9 balun i got (made a mistake ordering it). Also got some hyperflex 5 coax instead of the RG58 i was using. Think i'll be up and running wednesday next week.

73

PS: The new ham i got this idea from has made his first contact with Italy, 59 with 80W.

Someone has suggested the length is not ideal so I would follow that advice and somehow add length in a safe way. Extra aluminium rod off another silver rod at the top somehow drilled through and bolted ?

"PS: The new ham i got this idea from has made his first contact with Italy, 59 with 80W."

You could do that with a ham stick and 5 Watts on the right day and time it means nothing about this being a good antenna.

Though it cannot be worse than your ATAS whip + tiny radials.

As long as you are enjoying yourself is the main thing and you don't expect too much.

Good luck.

There are two opinions in the RFDX club. One has made the silver rod 5.35 m which is 1/4 WL on 20m. The other has lengthened it to about 7m. Both work fine, so i think i just do what works best for me and the MFJ-925.
I agree it should anyway be better then the ATAS

73
Karel

The KB2 9:1 UNUN  came in today. Now for the next stupid question: How do i know which connection is ground and which one radiator? There are no markings and i don't like opening it up to prevent messing up the waterproofing. Since it is a transformer DC measurement of continuity also makes no sense.

73
Karel

Have you got a picture of it.
Normally it's the terminal on the right for the ground or cp well that is how i make them and seen many the same,

 Hope this works

Would still go for the one on the left for the cp.

Alan Pilot wrote:

Would still go for the one on the left for the cp.

You mean right i suppose?

LOL yes.
Plot and lost spring to mind here.

A €10 electrical multi meter with continuity test would be useful here.

This is going to intentionally sound very wrong.

Stick your probe up the hole of the SO-239 then your other probe on the ring (outer 239).  

In continuity mode or resistance (Ohms) and watch it drop after checking continuity works by putting 2 probes together.

And it will show 0000 dead short.

You're right Alan. A very good multimeter might show a difference in (mili?)ohms between outer coax and ground and outer coax and radiator.

Hooked it up and it tunes on 7Mhz, not at 14, not at 28. Is this an indication of connecting ground to radiator and vice versa?

What is the exact length of wire you have on it.
LOL did you try the wire on the other terminal.

This is the setup, a piece (about 20cm)of 450 ohm ladder line between the KB2 and the antenna connectors, and an RF choke beneath the KB2 unun. I'll try changing the connections today.

I was too busy making bad jokes so just for correction of course continuity test can find out which terminal is connected to the SO-239 pin hole by having 1 probe in the hole and one on either of the terminals to see which it is connected to.

You could have bought a 12m fibre glass pole added X meters of wire for a radiator and radials, used your Yaesu ATAS at feed point and had a working antenna by now. The fact it does not even tune on a band the antenna could be designed to work on means you need to go back to the design/drawing board I think.

I do understand the experimentation side of ham radio but on the other hand making bad antennas and not getting any contacts does not make much sense to me. It goes for all the bad types of antenna one can make and there are a lot of them.

If you enjoyed it and are still satisfied then fine. If not, probably time to look at tried and trusted designs if the goal is contacts.

I understand your point. But i have some restrictions to work with, being the XYL who's afraid of thunderstorms, so the antenna must be able to lower fast, without too much threads hanging around. Second, i don't have that much space and have to consider my neighbours. Having said this and as you also have followed my experiments with the ATAS i'm slowly getting suspicious about the quality of the tuning/SWR part of my FT-897d.

edit: I had the W3EDP junior up and running on a 12m spiderpole, but the weather we had this autumn (september 2023 until now    ) made me realize how much clamps there are on a spiderpole and folding and bending this wire antenna made me look for a more sturdy solution.

The design/drawing board option has worked i hope. What am i building? The Silver rod, together with the aluminum pole is in fact a vertical dipole. Vertical dipoles need to have the feedline come to it at an angle of 90 degrees. Could that be the solution?

It tunes! As i know the radio enthusiasts being very fond of abbreviations, here's a new one. I think i now have an off center fed unbalanced vertical dipole or OCFUVD for short

And now seriously, the angle(90) of the coax feedline gave some better tuneability. Then i thought it strange having a dipole with one leg on the ground, so i lifted the whole thing about 50cm, which improved it even more. Now i'm going to watch the football game against France and tomorrow i think i'll:

- lift the contruction even more (can go to 1.3m above ground)
- try and also get the wires from the RF choke and UNUN to the feedpoint also at an angle of 90 degrees. I have some thick walled PVC pipe laying around which might be sturdy enough.

73 and thanks for all your input/comments/jokes.
Karel

FreqFreak wrote:

I was too busy making bad jokes so just for correction of course continuity test can find out which terminal is connected to the SO-239 pin hole by having 1 probe in the hole and one on either of the terminals to see which it is connected to.



If you enjoyed it and are still satisfied then fine. If not, probably time to look at tried and trusted designs if the goal is contacts.

This is a diagram of a 9:1 BALUN. If you look at it you'll notice that the red wire connects to the green wire and so does the blue meaning that if you do a continuity test with a multimeter it'll show a dead short across the PL259 and show the inner connecting to both terminals and the outer connecting to both terminals. 

Remember that something seen as a short at DC isn't always a short to RF.