Paul,

That antenna is very easy to model, and I get results from EZNEC within a fraction of a dB of the ones published on the manufacturer's web site, so I'm pretty confident the modelling is "telling the truth".

On 40m it is about one quarter the length of a full-size half-wave dipole, which means it will have a very low radiation resistance and very high capacitive reactance. EZNEC predicts 6.5-j780. If we assume the balun is a 4:1 (things are even worse if it is a 9:1) the impedance at the end of the coax will be 1.6-j195 which is an SWR of over 500:1; the losses in 50ft of RG213 would be about 11dB.

In addition to the cable losses, the problem with such extreme impedances is that the balun will never be able to prevent current flowing back down the braid, and so the coax becomes an integral part of the radiating structure. Changing the coax length, or what terminates it at the shack end, will significantly change what is now a very complex radiating structure, which is why you are seeing the variations you are.

It's a simple fact that in order to force 100W into that dipole on 40m you would need to develop 4.3kV peak across the feedpoint. You and I know that isn't happening, so we know the power must be being radiated by something else.

73,
Steve G3TXQ

Steve,

Below are the basic readings on my mfj 259B.Not sure about coax loss,reading taken when pressing the gate after taking the r & x readings.


With exra 12 ft of coax

Coax loss. reading with antenna connected
7.100........R = 7.......X = 53.....SWR......= 12.4.......................................0.6db
14.200........R = 65.......X = 95... SWR......= 4.9........................................1.7db
18.100........R = 72.......X = 58....SWR......= 2.7........................................3.3db
21.200........R = 70.......X = 59...SWR.....=.. 2.7........................................3.2db
24.900........R = 24.......X = 27...SWR.....=...2.7........................................3.2db
28.500........R = 19.......X = 11....SWR....=...3.1........................................2.8db

Without extra 12 ft coax.

7.100.........R = 3......X = 12....SWR...= 14.0.....................................0.6db
14.200........ R = 10......X = 2....SWR...= 5.0......................................1.7db
18.100........ R = 24......X = 25....SWR...= 2.6.......................................3.4db
21.200.........R = 20......X = 12....SWR...= 2.7.......................................3.2db
24.900.........R = 26......X = 30....SWR...= 2.8.......................................3.2db
28.500.........R = 23......X = 32.....SWR..= 3.2.......................................2.7db

Steve i am now trying to imagine how the figures would compare using ladder line rather than balun & coax,shame i recently gave my ladderline away not long ago or i would have tried it today.

73s
Paul.

Paul,

How long is your coax run, and what type is it?

Steve G3TXQ

Let me take a wild guess ..... 74ft of RG58 ?

Steve G3TXQ

78ft RG-213, give OR take a few inches Steve.

Paul.

Is that with the 12ft or without?

Steve G3TXQ

With the 12ft Steve.

I can only get close to your SWR and impedance figures if I assume a bit more loss than is accounted for by just the RG213 - that's why I guessed RG58 - but of course I don't know what the balun losses are.

The coax loss figures then are:

40m 13.3dB
20m 2.6dB
17m 1.2dB
15m 1.3dB
12m 1.8dB
10m 2.4dB

In comparison, these would be the losses with 78ft of Wireman554 ladderline:

40m 7.8dB
20m 1.3dB
17m 0.7dB
15m 0.5dB
12m 0.5dB
10m 0.6dB

The 40m figures are very suspect because, as I keep saying, I doubt it's the dipole alone that is radiating.

If I were building one for myself I'd probably use the ladderline option - the losses are slightly lower and it would save me building a remote balun. But if I already had the antenna, I doubt the savings would make it worth changing to ladderline.

73,
Steve G3TXQ

Thanks for the figures Steve,I will leave the ladder line for the time being.The antenna seems to have a very low noise floor considering my noisy enviroment & it is working o.k for me at present.

73
Paul.

You don't need a lot of space and cash to do that, just a copy of Razzle and a box of tissues.