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 :)
From: David Gilbert <firstname.lastname@example.org>
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?
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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