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Author Topic: A KLM KT34A in the Attic?  (Read 3841 times)
W6FG
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« on: March 13, 2011, 04:32:15 PM »

Here in the small coastal town of Morro Bay California, the view of the Pacific and the 'ROCK' is the all-mighty BIG DEAL!  Therefore, I counted myself lucky when no one complained when I installed a wire dipole above the roof of our tiny two-story house.  Anything over 25 feet high must go through the permit process. 

I have an old KT34A tri-bander disassembled in the garage. I'm sure that if I attempted to put it on the roof, the neighbors would squawk.  But, I think it might fit in the attic.  What is your opinion of how well that might work for me?  It would be a bit of a challenge getting the thing through the tiny ceiling hatch in the cosset of the bedroom up stairs, but if it would play and give me some gain it might be worth it.  I don't think there is room for it to rotate.  I guess it would just have to sit on the rafters pointing north/south.  Whatdayathink??
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WB6BYU
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« Reply #1 on: March 13, 2011, 04:59:02 PM »

It may be difficult to get it to perform up to expectations due to the surrounding
wiring / ducting / building detuning the parasitic elements.  I'd suggest just
starting with the driven element:  put it up in the center of the attic and see
what the SWR curve looks like.  You might get it to work as a dipole, then you
can try adding one element at a time to see if it makes any difference.  But
a yagi works though critical coupling among the elements, and there are lots
of things that can throw it off when the antenna isn't in the clear.
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WX7G
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« Reply #2 on: March 13, 2011, 05:32:01 PM »

BYU has a good idea. And you might stop at 2 elements. A 3 element Yagi-Uda offers less than 1 dB over a 2 element.
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WB6BYU
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« Reply #3 on: March 13, 2011, 07:05:08 PM »

Quote from: WX7G
...A 3 element Yagi-Uda offers less than 1 dB over a 2 element.


Well I suppose that depends on the specific design you choose.  I've got a 3-element yagi
design with 9.3 dBi free space gain, and haven't found a practical 2-element that does better
than about 6.8 dBi.

But regardless of the exact difference, using fewer elements isn't always bad, especially if
the original element spacing was compromised to accommodate multiple bands.  I used just
two elements of a tri-band yagi for a while with the full boom length because in that case
the original spacing was shorter than optimum on 20m (to allow it to work on 10m.)
Depending on the shape of the attic, if 2 elements allows greater spacing from ducting and
such, it may be a better choice than more elements.

But try just the driven element first to see how it works.
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W6FG
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« Reply #4 on: March 13, 2011, 07:50:24 PM »

Alright, good info!

Thanks guys I'll give that a try.  I hadn't even considered that the driven ele would tune up on it's own.  As an aside, I think I've heard of guys just laying a Yagi on the roof (mine is not flat so wouldn't work here) but I guess if it tunes, it can be used.

Thanks for the input,

rfg
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K5LXP
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« Reply #5 on: March 13, 2011, 09:30:41 PM »

> I hadn't even considered that the driven ele would tune up on it's own.

It probably won't, at least in my experience.   Any yagi I've played with relied on the parasitic effects of the other elements to arrive at 50 ohm feed Z.  At a minimum you'd need at least the reflector and possibly the first director to get it close.  In an attic, all bets are off until you try it.


Mark K5LXP
Albuquerque, NM
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VE3FMC
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« Reply #6 on: March 14, 2011, 08:36:26 AM »

> I hadn't even considered that the driven ele would tune up on it's own.

It probably won't, at least in my experience.   Any yagi I've played with relied on the parasitic effects of the other elements to arrive at 50 ohm feed Z.  At a minimum you'd need at least the reflector and possibly the first director to get it close.  In an attic, all bets are off until you try it.


Mark K5LXP
Albuquerque, NM


I have used the driven element from a 3 element tri-band and it tuned just fine as a single element.
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K5LXP
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« Reply #7 on: March 14, 2011, 09:13:33 AM »

You cannot take away a parasitic element and not affect the feed impedance of the driven element.  That's not to say you couldn't re-adjust an independently operated driven element to resonance, or to a desired match impedance but feed Z will definitely be different separately than when part of an array:

"The impedance at the driven element in a Yagi is affected not only by the
tuning of the driven element itself, but also by the spacing and tuning of
nearby parasitic elements, and to a lesser extent by the presence of ground."

ARRL Antenna Book, 20th ed, page 11-4


Mark K5LXP
Albuquerque, NM
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WB2WIK
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« Reply #8 on: March 14, 2011, 03:46:09 PM »

First, I'm astonished a KT34A will fit in the attic of a "tiny 2-story house."  It's a big beam.  It sure wouldn't fit in my attic even if I disassembled it into tiny parts and fully reassembled it up there: It still wouldn't fit, facing in any direction.

But anyway, before giving up on outdoor antennas in Morro Bay, you might look up Jerry W6TF.  He's in Pismo Beach and has a roof tower and large tribander, plus wire antennas and all sorts of stuff there.  I know that because I installed that stuff for him.  He's a District Attorney and may have some "pull" with the locals.
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N6EY
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« Reply #9 on: March 15, 2011, 08:58:37 PM »

One thing most folks missed here too is that two elements on the KT-34A are driven.

It's possible that it might work, but wiring and plumbing in the attic, as others have pointed out, will have a direct effect on the beam. 
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Jason N6EY
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N3DF
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« Reply #10 on: March 16, 2011, 09:43:22 AM »

The wire dipole above your house will almost certainly prove better.
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Neil N3DF
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