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 40m Single-Band semi-doublet 
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Post 40m Single-Band semi-doublet
I think this will work, but I will just run it past the gods before I buy the bits.

Problem:
I can't quite fit a 40m half-wave into my garden. But according to Mr Pythagoras, I should be able to get fairly close, about 28-29 feet per leg, tied-off to a six foot wooden garden fence.

Solution ?:
There is a black plastic drainpipe in the corner of my house in exactly the right place, just at the midpoint. The result will be an inverted 'V', but it will be a 'V' in both planes i.e. it will be a vee from above as well as from the side.

Now, as the antenna is only for 40m high-angle, I don't want to go down the multiband half-G5RV route. Instead I want to run as much dipole out as possible, then run the inner ends down opposite sides of the 80mm dia. drainpipe, secured with plastic ties, install a 1:1 balun at the current node, then carry on to the shack with coax, again sneakily hidden behind the drainpipe.

Is it that easy? i.e. add missing dipole leg length to parallel section minus VF?

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Sat Jan 19, 2013 3:11 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
I understand what you are doing in a sense but I cannot visualise how your antenna will run so a little clarification would not go a miss?

And if I teach you to suck eggs then slap me but I will help if I can :lol:


Sat Jan 19, 2013 3:39 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
Well, what you end up with is a dipole a bit less than half-wave. If each leg of the dipole is, say, six feet too short - unfortunately the 'wrong' six feet - then you run about six feet of open feeder down from the dipole. Roughly six feet down you can hunt for the current feed point, where you can tap off a rough 50 Ohms.

My question really is how much feeder can you get away with before the fact that it is open twin starts to 'take over', and you need a different ratio balun?

Remember, it is a single-band antenna.

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Sat Jan 19, 2013 4:04 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
So you are wanting to physically shorten the driven elements so to speak and take the excess miss match with the open wire feeder acting as matching stub as it does naturally with a 4:1 current balun? Please tell me I am on the right path lol.

So is there no way of accommodating 20m to allow you on 40m and "T" it Marconi style for 80m using the open feed line as part of the matching and radiating element?

Just trying to keep it simple and not over complicate the issue as that is what tends to happen.


Sat Jan 19, 2013 4:18 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
Thanks John. Not really bothered about 80m now, cos the clocks will be going forward again before long. Next winter I'll use one of my catapulted contraptions if I want to use 80m after dark.

That's the question. 1:1 balun because you are looking for a 50 Ohm feedpoint?, or 4:1 because it's a short length of open twin?

For summer, I've settled on just three bands: 40m with the 'doublet', 30m with a fishing pole vertical and 17m with a loft 'V'.

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Sat Jan 19, 2013 4:28 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
Well I would have said a 4:1 balun as I have tried both with my doublet(s) and the 4:1 gives you a little more flexibility in the matching, but then I have to go against my choice as you are using it as a mono band doublet and hence a small bandwidth to match so maybe the 1:1 is the choice.
I will be interested to see what Steve recommends and to verify if the knowledge I have absorbed is actually starting to be used with such discussions like this.


Sat Jan 19, 2013 4:35 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
can you not use ladderline as a matching section cut at the 50ohm point , 1.1 balun and coax to shack ?.. job done ..


billy

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Sat Jan 19, 2013 4:36 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
m0jha wrote:
can you not use ladderline as a matching section cut at the 50ohm point , 1.1 balun and coax to shack ?.. job done ..


billy


I'd rather use wider spaced, black wire running down opposite sides of the 80mm drainpipe. That makes it a proper 'secret agent' job :)

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Sat Jan 19, 2013 4:40 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
you still can but you probably won't know the caracteristic impedence of it ( not that it will matter ) cut it at the 50 ohm point and balun/coax to shack.

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Sat Jan 19, 2013 4:44 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
m0jha wrote:
you still can but you probably won't know the caracteristic impedence of it ( not that it will matter ) cut it at the 50 ohm point and balun/coax to shack.


Thanks Billy. 1:1 it is. Going to send off for it tonight.

Was that you beeping under the big net? I put a quick call in morse after the end, but my rig goes to USB on CW, so I had to wangle it. :)

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Sat Jan 19, 2013 4:50 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
i thought someone may think it was me ... No i tuned up to the net on my HB1B as the 817 was not hooked up ,i was listening for about 1 min when i heard the morse .. i wouldn't frig about doing that mate ... when i went back down i was sending cq on LSB like a wally for a few mins before i realised i hadn't changed back to a real mode :)

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Sat Jan 19, 2013 5:01 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
m0jha wrote:
i thought someone may think it was me ... No i tuned up to the net on my HB1B as the 817 was not hooked up ,i was listening for about 1 min when i heard the morse .. i wouldn't frig about doing that mate ... when i went back down i was sending cq on LSB like a wally for a few mins before i realised i hadn't changed back to a real mode :)


I don't think anybody would have been upset. I just thought 'Ey up, Billy's lurking'.

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Sat Jan 19, 2013 5:06 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
Chris,

What you're trying to do makes sense! Here's some numbers:-

A 40m dipole that's 6ft short on each leg has a feedpoint impedance of about 43-j321 Ohms - in other words lots of capacitive reactance because it's short. If we now feed it with ladderline and try to find a current node we get:

5m of 300 Ohm line, 20+j0 Ohms, SWR=2.5:1
4.3m of 400 Ohm line (450 commercial), 26+j0 Ohms, SWR=1.9:1
3.3m of 600 Ohm (wide-spaced), 33+j0 Ohms, SWR=1.5:1

As you can see, the higher Zo line gives better results, so something wide-spaced inside (or outside) the drainpipe sounds good. In all cases a 1:1 balun is what you need at the transition to coax.

Your "V-shaped Inverted-V" will change the numbers some, but the principles are just the same.

Hope that helps,
Steve G3TXQ


Sat Jan 19, 2013 5:14 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
Excellent Steve, and thanks.

So that's the 40m NVIS antenna and the 17m 'Euro Howitzer' sorted.

Next payday it's Spiderbeam for the 30m vertical. Already got the copper standing by for the earth. So here comes summer :)

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Sat Jan 19, 2013 5:23 pm
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 40m Single-Band semi-doublet 
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Post Re: 40m Single-Band semi-doublet
Steve.
To save me going up and down the ladder all day, and then cutting too much off, am I right in assuming the following:

''Make the twin feeder too long, then snip bits off, then stop at 1.5:1, because it will not get better and will start to get worse if you cut any more off''. 1.5:1 is ideal, anything lower is 'false tuning'.

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Sat Jan 19, 2013 6:53 pm
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