For those of you using a single band resonant vertical consisting of a 1/4 wave vertical wire supported by a fiberglass pole plus radial (counterpoise), how are you feeding it? Do you feed directly with coax, do you use some sort of unun xfmr (please describe), do you use a coax choke (air wound or toroid wound?) etc etc???
I have a quarter wave vertical for 20m attached to a 7m fibreglass telescopic pole. At the feedpoint I have 3 radials, each 5.02m long, and the feedpoint is about 1.5m above ground level. The radials each have a section of cord at the distant end acting as an insulator, so the ends are in the air and not connected to ground in any way. I feed this with RG58 50 ohm coax, with no transformer, balun or unun. Its feed impedance is such that the measured SWR is about 1.1 on 14.1 and it rises slightly at 14.3 but not enough to affect the transmitter output power.
This antenna has provided many SOTA dx contacts using 10w of output power from an IC703, mainly European long path contacts.
I agree entirely. For my activations in Portugal I constructed a coaxial choke made with RG-174 to stop the RF coming back down the coax. I use a small 5m pole and feed the 14MHz 1/4 wave at just above ground level and deploy two radials (or should that be counterpoises - we’ve had that debate!). Before I constructed the choke I received some quite vicious RF burns off my straight key, even though the rig was just an 817. Bizarrely, I don’t seem to have the same problem when using the antenna in Scotland which must be something to do with ground conductivity.
As per Jonathan, my 12m 1/4wave GP consisted of 1/4wave vertical radiator and 3 of 1/4wave radial drooping at 45degs. It was fed direct with 3m of RG58 from the 817. I needed to adjust the vertical section marginally from 1/4 to get the match to 50ohm +/-. The radials ended in bulldog clips and were clipped to the guy cords. It all fitted on a 5m mast.
One of the reasons for a 12m challenge was I could fit a DX magnet of an antenna on my existing mast without buying any new equipment! Hey, there has to be some advantage for helping to run the scheme
I suppose that feedline length will be a factor too. I used a 1m patch lead from the antenna base for VSWR trimming, once I’d finished, I used 25m of RG-213 to connect the antenna to my rig. The antenna was built for 18MHz.
I wonder what would happen if fed without a choke and using a feed line length more closely related to the frequency of use?
I think the use of a common mode choke is a sensible idea.
Not matter how many times you call it a 1/4wave for 40 it’s not. 40/4 = 10m and your radiator is 10% too long. It’s not a 1/4wave for 40m with that length or radiator, those lengths of radials or that set of angles. A 1/4wave is resonant and can be setup so it doesn’t need any matching to 50Ohms.
You antenna is some random length end-fed with assorted impedance lowering (the 9:1 unun) and impedance matching (MFJ949).
It works as you have had an S2S with VK which is fine and dandy. But that still doesn’t make it a 1/4wave for 40!
Yes Karl your antenna is interesting! An electrical quarter wave of PVC insulated wire at 7.1MHz is approx. 10.2m long, then you’ve got that coil in the middle which will add some inductive centre loading so I wouldn’t be surprised if it actually resonates around 60m (would be interesting to actually check the resonant frequency)
If it were resonant on 40 with an impedance around 50 ohms or probably less, by the time you’ve allowed for the Unun you’d be down around 5 ohms at the feedpoint which I guess your ATU would struggle with so it’s probably a good job
I use a similar rationale for the doublet I use for most activations - it’s a bit longer than resonant on 60m, but gives very convenient settings on the ATU on 80, 60 and 40
i also transmit with 10…100W and feed the vertical (10m long) direct, without any filter, balun or unun. i had not any problems yet.
my packpack with the TRX (alinco dx sr8) stands direct close the vertical, no coax between antenna and tuner. no promlems any time