The QRPer Board

Brian, probably I oversimplified, what I mean is that one may like being able to jump on a non harmonically related band, so just by changing the EFHW length to a "good" one for a random and changing the tap to 9:1 we would then be able to jump on (say) the 30 meters band or whatever else, in short with such a double transformer we'd have two antennas in one, which isn't a bad thing imVHo

Andrew (grayhat) wrote: ↑Tue Jun 27, 2023 3:55 pm Brian, probably I oversimplified, what I mean is that one may like being able to jump on a non harmonically related band, so just by changing the EFHW length to a "good" one for a random and changing the tap to 9:1 we would then be able to jump on (say) the 30 meters band or whatever else, in short with such a double transformer we'd have two antennas in one, which isn't a bad thing imVHo

Hi Andrew,

Not over-simplified at all. What I was saying is that by using the 49:1 resonant length for all resonant bands (40, 20, 15, 10), and switching the same length to 9:1 for non-resonant bands (80,60,17,12), you can use the same radiator length for all.

Brian - K3ES wrote: ↑Tue Jun 27, 2023 4:09 pm

Andrew (grayhat) wrote: ↑Tue Jun 27, 2023 3:55 pm Brian, probably I oversimplified, what I mean is that one may like being able to jump on a non harmonically related band, so just by changing the EFHW length to a "good" one for a random and changing the tap to 9:1 we would then be able to jump on (say) the 30 meters band or whatever else, in short with such a double transformer we'd have two antennas in one, which isn't a bad thing imVHo

Hi Andrew,

Not over-simplified at all. What I was saying is that by using the 49:1 resonant length for all resonant bands (40, 20, 15, 10), and switching the same length to 9:1 for non-resonant bands (80,60,17,12), you can use the same radiator length for all.

I see your point now, well, I think it may be worth experimenting, if that works then one would just need to change the transformer binding post to the needed ratio and use the same antenna on multiple bands, not just on the harmonic related ones, I'm thinking to model a 40m EFHW (plus a 17ft counterpoise) in 4NEC2 and check the mismatch values, sure that will just be a simulation, but could offer some idea about what to expect

Ok, ran a quick test, built a NEC model with a 1/2 half wave (on 40m) radiator and a 17ft counterpoise, placed the radiator at 20ft meters from ground and ran a sweep from 1 to 30 MHz, then changed the impedance from 2450 (49:1) to 450 (9:1), the results are in the attached image below, the antenna stays under 10:1 from 11MHz up to 30MHz so a good ATU should hopefully be able to find a match, at least on 17 and 12 meters bands, adding some wire to make the antenna longer will improve things (when used as a "random"), same goes for counterpoise, the ideal length (tried it in NEC) when used as an EFHW is 7.2ft, while when used as a "random" it needs the 17ft counterpoise

Andrew (grayhat) wrote: ↑Tue Jun 27, 2023 1:05 pm regarding chokes, as I wrote, my reference design is the one from Rick (DJ0IP)

https://www.dj0ip.de/rf-cmc-chokes/diff ... lla-choke/

but that's for QRO and uses a quite heavy toroid (FT240...) which adds some weight to a portable kit, for QRP a 140 should suffice, going for a smaller one may probably be possible, but then there won't be room for the windings and due to the small size the windings will probably present inter-capacitance <sigh> anyhow, if someone is interested a good reference for CM chokes is this one

http://www.karinya.net/g3txq/chokes/

Then if one prefers buying instead of winding... LDG sells a decent choke

https://www.dxengineering.com/parts/ldg-ru-1-1

which can easily be connected at the output (coax side) of the 9:1 transformer

If you don't have a 4:1 or 9:1 around to try, just cut your feedline or add a few feet to it. Try trimming or lengthening the antenna or counterpoise by just a few feet. I have some short alligator clip lengths of 1 ft, 2 ft and 3 ft and I also have short lengths of 300 ohm line and coax. I can almost always get the tuner to handle things just fine by adjusting these parameters. (A 1:1 choke is really helpful at the feedpoint, too.)

Freyja
WA3JPG

Hi
I've been away for a while, so I missed the last part of this conversation. I had the same idea, only using a switched l-match type tuner to place the capacitor eitther in series or shunt to match high or low impedances. And I was gonna wind the antenna in to I think 57' for the random wire length..

Great minds thinking alike (sorta!)

Rick N8TGQ