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 40M/15M Dipole 
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Post 40M/15M Dipole
I've set up a 40M dipole about 6M above ground at one end and about 8M at the other. Initially, the antenna was resonant at 7.1 MHz with a 1.2:1 SWR at the radio. I've used 75ohm twin lead down to a 1:1 transformer balun (not a choke type), then 50Ohm COAX upto the shack. The maximum power to be used is 100W. My primary objective is to make the antenna dual band with no need for an ATU.

The purpose of using 75 twin is to keep the antenna weight down - it avoids the use of an airborne Balun with coax suspended from it. I suspect the antenna feedpoint impedance is probably 40-50 ohms (but I'm not an acrobat so can't get at the feedpoint!).

On 15M the system was resonant at 22.1MHz with a 1.1:1 SWR. The bandwidth between the 2:1 SWR points measured around 1.8MHz. Tonight I've added a couple of capacity hats, basically an "X" where each of the four legs is 6 inches long. They are roughly 1/4 wavelength either side of the feedpoint.

This has made no material difference at all to operation on 40M - which is great.

On 15M the system is now resonant at 21.55MHz, but there is some reactance creeping in and the SWR at resonance is rising toward 1.3:1. The bandwidth appears unaffected by the capacity hats.

Question 1 is - if I make the capacity hats bigger to pull the 15M resonance down to say 21.2MHz is the reactance going to increase (and therefore the SWR worsen)? It seems that way. This would suggest that the use of the capacity hats is trading the feedpoint impedance against the electrical length of the antenna, so at resonance I won't see pure resistive impedance, but a good dose of reactance too. (On 40M I'm not seeing any reactance at resonance - which is what I'd expect)

Question 2 is - what impact will moving the capacity hats up and down around the 1/4 wave points have on the feedpoint impedance?

Steve
G0AIN

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Steve
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That looks interesting - I shall poke it with a stick...


Tue Dec 10, 2013 10:19 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
Steve,

By definition resonance is where X=0, so you can't have any reactance at resonance. I think what you probably mean is that you have some reactance at minimum SWR - that's a different thing!

I haven't modelled it, but I would expect the better option to be shifting the position of the hats rather than making them bigger.

Steve G3TXQ


Tue Dec 10, 2013 10:38 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
Steve,

Well - I have now modelled it and I was wrong! Shifting the position by quite large amounts has little effect on the resonant frequency, so I think you're stuck with increasing the size of the "hats".

Did you realise that when it's operated on the third harmonic the dipole has a "natural" resonant impedance close to 100 Ohms; that will then get transformed by your 75 Ohm section to something quite different at the balun. So hoping for a very low SWR on 15m is probably a bit unrealistic. But if you could make the 75 Ohm section a quarter-wavelength on 15m you might get very close!

Steve G3TXQ


Tue Dec 10, 2013 10:51 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
Thanks Steve,

I solved my schoolboy error on the resonance issue - I took the measurement where I wanted resonance to be not where it was!

I think the plan is to lower the resonance on 40M by about 50KHz, which will be around 150KHz on 15M. Then if I adjust the 75ohm section to improve the match as you suggest, then I'll be pretty close to the objective. I can then adjust the capacity hats for final tweaking.

Steve
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Steve
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That looks interesting - I shall poke it with a stick...


Wed Dec 11, 2013 7:45 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
I'm pretty sure now that's the best way to do it: stick the "hats" a 15m 1/4 wave either side of the feedpoint; adjust their size for minimum SWR at the required frequency; and alter the 75 Ohm section for best match. The last two steps will probably be iterative.

I'm sure you know that simple lengths of vertical wire would do instead of the "hats".

Good luck, and let us know how it goes.

Steve G3TXQ


Wed Dec 11, 2013 9:06 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
Surely a simple coax-fed half-wave 40m dipole is also resonant on 15m too?

A simple shop-bought 1:1 at the feedpoint and the cheapest ATU on the market, to just tune-out the 'three times' bandspread on 15m surely?

I just don't understand the over-complication ... the need for the hats etc. An antenna that low will probably 'show' more like 75 Ohms anyway, so no need to even change the coax.

Is it me?

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Wed Dec 11, 2013 9:32 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
It's a common misconception that a dipole which is half-wave resonant at, say, 7.1MHz would also be 3-half-wave resonant at 21.3MHz. It's not - it's usually resonant up around 22MHz, and the SWR at 21.3MHz would be 6:1!

The standard "fix" is to add some capacity at points that are 15m quarter-wave either side of the feedpoint.

Steve G3TXQ


Wed Dec 11, 2013 9:47 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
G3TXQ wrote:
It's a common misconception that a dipole which is half-wave resonant at, say, 7.1MHz would also be 3-half-wave resonant at 21.3MHz. It's not - it's usually resonant up around 22MHz, and the SWR at 21.3MHz would be 6:1!

The standard "fix" is to add some capacity at points that are 15m quarter-wave either side of the feedpoint.

Steve G3TXQ


True Steve ... I'm just being 'a little bit country' again. But if you were to cut the antenna for the 'mid point', where the SWRs were 'equally bad' on each band, then the losses would be negligible with a small coaxial ATU ??????

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Wed Dec 11, 2013 9:50 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
Yes, you can do that. If you were a "CW man" you could cut it for resonance at about 6.8MHz; you'd then have an SWR of around 2.4:1 on both 7.05MHz and 21.05MHz.

Steve G3TXQ


Wed Dec 11, 2013 10:00 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
G3TXQ wrote:
Yes, you can do that. If you were a "CW man" you could cut it for resonance at about 6.8MHz; you'd then have an SWR of around 2.4:1 on both 7.05MHz and 21.05MHz.

Steve G3TXQ


That's how I would do it. But I am the 'dodgy antenna man'. You can give him the 'perfectionist' option :)

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Wed Dec 11, 2013 10:03 pm
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 40M/15M Dipole 
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There's no "perfectionist" about it!

I simply read his original post which said: "My primary objective is to make the antenna dual band with no need for an ATU". Perhaps you missed that?

He'd already decided on a particular approach and was asking for advice on how to optimise it.

Steve G3TXQ


Wed Dec 11, 2013 10:07 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
G3TXQ wrote:
There's no "perfectionist" about it!

I simply read his original post which said: "My primary objective is to make the antenna dual band with no need for an ATU". Perhaps you missed that?

He'd already decided on a particular approach and was asking for advice on how to optimise it.

Steve G3TXQ


No. I heard it Steve. It just seemed to me that the benefits to be gained by all that cap-hat loading etc. were not worth the effort compared to accepting a moderate SWR that most modern rigs can get rid of anyway.

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Wed Dec 11, 2013 10:11 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
You two need to put the issues behind you and work together as both can aid the newcomer as well as the advanced amateur radio operator to build optimum antennas!


Wed Dec 11, 2013 10:12 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
I don't know whether his radio has an internal tuner or not - he didn't say; and if not, 4ft of extra wire will certainly be less expensive than an external tuner.

When I see a post as well-informed as Steve's, I tend to want to encourage them in their experimentation, not suggest other solutions.

Steve G3TXQ


Wed Dec 11, 2013 10:16 pm
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 40M/15M Dipole 
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Post Re: 40M/15M Dipole
2E0JGR wrote:
You two need to put the issues behind you and work together as both can aid the newcomer as well as the advanced amateur radio operator to build optimum antennas!


I have no issue with Steve. None whatsoever.

Steve has kindly helped me with a technical query in the past, and if I need a cold calculated technical answer to a question that is beyond my knowledge, I will ask him again.

Please don't try to create a problem where there is none. Also please appreciate the BIG difference between 'debate' and 'argument'.

Friends debate ... enemies argue.

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Wed Dec 11, 2013 10:17 pm
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