[TowerTalk] balanced line loss on a mismatched antenna

[Gary Slagel]

Thanks for all the feedback!
 
As far as reasons for trying the 8JK.... Mainly I want a 40M antenna that I can put on the same mast with my hf tribander.  The 8jk is nice cause it has some gain and I could also tune it for the WARC bands.  I like that its a non-resonant antenna so I wouldn't worry as much about interaction with the tribander (that's a whole nother thread... would a 40M yagi cause me problems 6' away from my tribander).  
 
I also thought Cebik's idea of 44' elements was to use it on 10 thru 40 without the lobes splitting up on 10 but that was for a single element.  Maybe his 44' 8jk is a 10 thru 30 deal.   Anyway... I agree, no reason to put the thing up if you're going to have 3db or more loss on the feedline.  
 
I'll look at TLW when I get home.  I'm still curious if, using a different length of feedline, I can get the SWR down below the 248:1 or 375:1 that you mentioned in your other post.  Is it reasonable to think that by varying the length of the feedline I can bring a huge mismatch like that down to 10:1 or less?  And if I could, would that reduce the loss?
 
Starting to look like one of those lotsa pain no gain projects :)
 
Gary Slagel
KT0A

 


________________________________
From: David Gilbert <xdavid at cis-broadband.com>
To: towertalk at contesting.com 
Sent: Tuesday, February 28, 2012 1:07 PM
Subject: Re: [TowerTalk] balanced line loss on a mismatched antenna



I guess I'm still confused what there is to gain by building a W8JK 
instead of something like a 2 element yagi.  Mechanically it's at least 
as troublesome as a yagi since the W8JK has two feedpoints.  
Electrically it has negligible additional gain over a yagi, has way 
lower feedpoint impedance, and has zero front/back.  The only advantage 
to a W8JK is that you don't have to adjust the lengths since you feed 
both elements directly and control the current phases that way.  I'd 
bet that it is way more than coincidence that most commercial 
manufacturers have settled on coil-loaded shortened reflector/driven 
elements with a hairpin match at the feedpoint for 2 element 40m gain 
antennas.  The W8JK does allow pretty close spacings (i.e., short boom), 
but if that's the objective you can do as well or better with a 
director/driven element configuration.

Can anyone explain to me why it would be preferable to force feed a W8JK 
design into this situation?

73,
Dave  AB7E




On 2/28/2012 12:04 PM, Paul Christensen wrote:
>> "You'd be way better off with just a dipole (EZNEC says 22 - j600 ohms for a 44 foot element) and either..."
> I agree with Dave's loss numbers for 44 ft element length.
>
> However, before giving up and using a shortened 44 ft. dipole, you may want to see if there's a way get the elements lengthened close to a halfwave and still keep spacing at 0.1 wavelength.  Perhaps bring the ends down if you're maxed-out on horizontal space.  If you can, then you can use my prior message as a guide for system loss.  I would still model it with the hanging ends.  Otherwise, the 44 ft length on 40m is going to be a real looser with the 8KJ as the added element really drives the impedance down from mutual coupling and the short element length makes it too reactive to keep losses low, even when trying to match at the feedpoint.
>
> Paul, W9AC
>
>
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