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 Cobwebb with 40 Metres - interaction with 15 Metres 
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Post Cobwebb with 40 Metres - interaction with 15 Metres
Good Morning,

I am corrently looking at building a "wires only" Cobwebb type antenna in-situ in my loft. All looks very do-able and indeed it seems like I coud actually add a 40 metre element to the design within the space available. This would be a nice to have, even though I am mainly interested in the higher bands.

The standard design of course covers one octave - with the addition of 40 I would be covering two octaves. This brings with it the issue that the 40 metre element will also be resonant on 15M.

One option would be to remove the 15m element. The 40m element should work ok, however will 15m performance suffer as a result of using an antenna over one wavelength long? Will the radiation pattern be different? Is there anything in particular to consider given the Cobweb's folded elements?

Or what if I leave it in place? Ignoring the interaction between elements, the feed impedence will presumably be lower but fairly tame, as it will consist of two resonant (and hence predominantly resistive) antennas in parallel. But what of the interaction between elements and what on earth will the ratiation patern look like?

Lots of other variations of course (traps in 40m antenna, seperate feed via a diplexer etc.) but you get the idea.

Thoughts much appreciated.

Regards, Rob


Tue May 19, 2015 7:27 am
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 Cobwebb with 40 Metres - interaction with 15 Metres 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
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Post Re: Cobwebb with 40 Metres - interaction with 15 Metres
Hi Rob,

You wont easily be able to use a 40m element on 15m - you'll have an impedance of around 12 Ohms at the 40m resonance, but 150 Ohms at the 15m resonance. That's because on 15m the inside half-wave of the current standing wave pattern is relatively "unbent" and it behaves more like a linear 3/2 wave. If you employ the usual 1:4 matching on 40m (either folded-dipole or 1:4 balun), you'll have a 12:1 mismatch on 15m.

One option would be to feed the 40m element separately without any matching, and accept the moderate SWR on 40m and 15m, provided the coax run was not too long and lossy. You would of course need a tuner on both bands

A 40m element used on 15m begins to be less omnidirectional, but it's not extreme and is probably comparable with the pattern changes you might experience from other "clutter" in the loft.

Hope that helps,
Steve G3TXQ


Tue May 19, 2015 8:06 am
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 Cobwebb with 40 Metres - interaction with 15 Metres 
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Post Re: Cobwebb with 40 Metres - interaction with 15 Metres
Thanks Steve. That makes sense.

So if I install both 40m and 15m elements and connect to the same feed (with tyhe usual 1:4 matching), things should be moderately ok? When operating on 15m, the 40m element will present a significantly higher impedance than the 15m element (and the 1:4 balun output) and therefore it will have limited effect on return loss and little current will flow in it. Right?

I guess this is not too dissimilar to those adding a 6m element even though the 17m element is not so far from resonance on 6m.....

Regards, Rob


Tue May 19, 2015 8:27 am
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 Cobwebb with 40 Metres - interaction with 15 Metres 
Silent Key

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Post Re: Cobwebb with 40 Metres - interaction with 15 Metres
Rob,

Yes, the 40m element should look like a high impedance on 15m, so the 15m element should still take the vast majority of the current.

I'm not sure what matching arrangement you plan to use - folded dipole or 1:4 balun. If you opt for the balun you'll need to make sure it works OK down at 7MHz.

Steve G3TXQ


Tue May 19, 2015 9:22 am
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 Cobwebb with 40 Metres - interaction with 15 Metres 
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Post Re: Cobwebb with 40 Metres - interaction with 15 Metres
Yes, planned to use the Balun. In the past when plaining with UNUNs I have constructed two identical ones and then connected back to back in order to measure insertion loss (luckily have acess toa VNA at work).

But in this case I might just wing it on the grounds that if there is a problem I will either be unable to get a good return loss on 40 metres or the balun will get unbelievably hot when I transmit.....

Many Thanks,

Rob


Tue May 19, 2015 10:55 am
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