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Tuning a 20m GP for 30m


#1

This afternoon I had an hour to spare so decided to set up my FT817ND and portable 20m GP antenna in the garden. I was very pleased with the results with contacts around Europe. I’d like to be able to use this on 30m and I know I could use a loading coil at the feedpoint. But could I put the coil right at the back of the rig before the coax? I know this won’t be the most efficient way to get on 30m but it could be an easy way to change bands without having to take down the antenna.


#2

Depends on the lenght of the cable. A shortened antenna like a 20 m antenna used on 30 m has a capacitive reactance at it’s feeding point as a part of its impedance and is of course not “tuned” to 50 Ohms. A 50 Ohm transmission line with this kind of load transforms the impedance of the load depending on the lenghts and repeats this transformation at intervals of half an electrical wavelenght. Assuming a lossless transmission line the impedance changes while SWR remains constant. Lossy transmission lines show lower SWR the greater the distance from the antenna is.

The change of the impedance along a transmission line leads to the fact, that at the transmitter-end of the line a great variety of impedance-values can be expected, including values with inductive reactance. These inductive reactances can only be cancelled out by a capacitive tuning element, a coil would lead to an even worse efficiency of the whole system. So if you don’t want to use a loading coil at the base of the antenna you have to use an antenna tuner between transmission line and transceiver. A tuner can reduce either capacitive or inductive reactances. In this case the transmission line will have higher losses especially on the higher HF bands, so the line should be as short as possible, especially at qrp-levels.
73, Alexander, DL1AIW.


#3

Hi,

You could chose a length of cable electrically equal to a half wave on 30 m and switch in a resonating inductor for 30 m. But that is not what is usually done as you know. The reason is of course it doesn’t work nearly as well. You cable will have a high SWR and hence higher losses and you may well get rf on the outside of the coax leading to rf feedback and pick tuning.

If it were a good solution we would all be doing it.

73
Ron
VK3AFW.


#4

This!


#5

No. Well, you could but it would not do what you want. The feeder is not part of the antenna in this sort of design, so it won’t affect the resonance. The radio has a 50R output, connecting 50R coax to it is just extending the socket and has no effect on the match.

If you want to do something without taking down the antenna then a base loading coil is pretty much your only choice unless you go for something complicated like a parallel Marconi


#6

Just a thought: cut a length of 300 ohm ribbon to a quarter wave on 20m, then cut one side about a third of the way up and insert a loading coil to bring it to resonance on 30m, both sides of the ribbon go to the centre wire of the coax. You would need two different lengths for the GP of course or just insert traps for 20m and the overall length resonant on 30m. Alternatively if your pole is long enough just have parallel vertical wires, a piece of twin ribbon with one side cut back to resonate on 20m and the full length of the other side resonating on 30m.


#7

It depends… In theory the impedance presented to the radio on 30m is calculable by taking the capacitively reactive impedance at the base of the 20m quarter wave, then transforming it by whatever electrical length the feedline is on 30m, to arrive at an impedance expressed as Zin = R + jX, where X may be negative as it was at the base, or may have been transformed to a positive (inductive) impedance, at the rig end of the feedline. The special case is where the feedline is a (multiple of a) half wave on 30m, allowing for velocity factor, where the same impedance that was present at the antenna base is presented to the rig. Some ATUs might be able to cope with the reactive load. An L match would be able to transform the impedance to 50 ohms, however the conventional view of such situations is that the SWR will be high on that cable, increasing losses. it would be better to put the L match at the base of the antenna. Depending on how tolerant you are of losses, it might be usable but certainly not ideal. As Ron says, it it was great, everyone would be doing it. And there would be a bright shiny one offered on ebay.

My approach to a 20/30m vertical is to insert a suitable link in the 30m length to allow it to be the correct length on both bands. You have to drop the antenna to change bands, but that’s not arduous. The radials can be set up in advance to have two for each band so would not need to be link-switched but the antenna may well work better with 3 correct length radials for each band. I found quite good performance on 30m using the 3x 5m radials that were used on 20m without change. I haven’t compared that setup with 3 correctly sized radials on 30m.

73 Andrew VK1DA/VK2UH


#8

Exactly what I do for my 17/20m vertical. I have power pole links in the radiator and all 4 radials. Okay, I have to get up drop the antenna briefly to set the link on the radiator and then stroll around the radials to set those, but it takes no more than a minute or so and my legs usually enjoy the exercise! Must be getting old. :wink:

73, Gerald G4OIG


#9

Thanks for your replies. At the weekend I used my ATU to match the GP to 30m. It tuned fine but I didn’t get any QSOs. However, it was only for a short time plus on the antenna’s resonant band I find QSOs a bit hit and miss anyway. On 20m I had a nice QSO with a Russian station who gave me 559 but I couldn’t get a Greek station to hear me. Unfortunately the RBN was down at the time so I couldn’t use that to see what strength I was being received at on 30m. I will try this again soon.


#10

About a year ago I reconfigured my 20m GP antenna so that a little ATU could be slotted in at the feedpoint. I was then able to tune it to several different bands, but getting QSOs seemed to be unusually hard work. So possibly similar to your experience.

Shortly after that my mind became clear: QRP SOTA works best with single band resonant aerials, not compromise antennas.

I am thinking about a 30m GP antenna. I would simply scale up from my 20m GP and use a 10m travel mast.