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Author Topic: W6TC compact Quad loop for 80&40meter's, anyone ever use this antenna  (Read 2169 times)
KE2TR
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Posts: 130




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« on: July 17, 2013, 06:47:29 AM »

Back in the 70ies when we all had used handbooks for antenna's or whatever I had picked up a set of Bill Orr's handbook with all his antenna book's. For year's before we all started to use the net they were my Bible's of info and go to info, I never threw them out and still read them from time to time, hey they bring back the old day's of antenna building which for me was the most fun. I just recently move qth's and while the tree guy was here pruning some big oaks and white pines that were way overgrown I had him place some lines in spots that seemed sturdy and were around 75' high. I ordered up some wire, rope and 450 ohm HD ladder line and am all ready for a hot day of antenna building but I opened up Bill's Antenna book and had seen this quad loop antenna but W6TC, the loop is 70' top and bottom and 35' each side, the feed point is in the center of the bottom leg and its 450 ohm line for about 35' then goes to coax, you could also place a 1/4 wave 75 ohm coaxial stub in line for 40 meter operation. The total wire used in the loop is 210' then the 35' of open wire line which you use to tune the antenna for what part of the band you want. I would get the top of this antenna at 75' and the bottom at 30' and was wondering if anyone out there has ever tried this antenna and with what results. I have tried delta loop before but at low heights and to be honest they have all sucked but they all were close to the ground and my old qth was on LI,NY which was all friggin sand but here it seems the soil is better, well the amount of earth worm's here tells me it just might be slightly better with high mineral content. There was also another antenna which is a 135' center feed dipole with open with line as well going into a 4:1 balun and then a transmatch which would be at 75' as well but the end's may have to be bent down since I don' have the total wing span of 135' but maybe 100'. When I looked up a rectangle shaped loop on W4RNL's site it seemed to model almost like the center fed dipole as far as gain and use on the higher band's with a transmatch but I really want the best operation on 40&75mtrs, the higher band's would be a plus but not a deal breaker. Any comments please, thanks.
Jim
KE2TR
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WB6BYU
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Posts: 13038




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« Reply #1 on: July 17, 2013, 09:08:57 AM »

Quote from: KE2TR

... I really want the best operation on 40&75mtrs...



What distances do you want to cover on those bands?

The W6TC design isn't bad - it works like an end-loaded dipole on 80m, and a 3/2 wave
loop on 40m.  One issue will be getting the resonances to line up in the desired spots
on both bands, though you probably could feed it with 75 ohm coax and use a tuner
in the shack with reasonably low losses.

I'd have to model it to see how it compares to using a 80m doublet (with the ends hanging
down) at the full 75' height.  I suspect that the doublet will have a slight advantage on
80m, while on 40m the doublet would have a narrower azimuth pattern with a greater average
height, while the loop would have a wider pattern with a bit of broadside gain (though not a lot
with the sides spaced only 1/4 wavelength.)  The doublet would need to be fed with open wire
line, while the loop could have coax from the end of the open wire line to the tuner.

But overall the differences between them probably aren't too much, and there aren't any
glaring flaws that I can see offhand in the W6TC loop design.  If it fits your needs better
than some other options, then go ahead and give it a try.


I encourage hams to experiment with different antennas to find what works for their own
specific situations.  For such temporary antennas I usually use stranded, insulated hookup wire,
or whatever you can find locally that is relatively cheap.  Once you find a design you like then
you can consider building a more permanent version, as the plastic insulation on most hookup wire
degrades over a couple years in the sun and small wires are more prone to breaking.  But it is
light, convenient to work with, and easy to reuse in later antennas.
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KE2TR
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Posts: 130




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« Reply #2 on: July 17, 2013, 08:46:49 PM »

Looking for a DX and rag chew antenna, had a pair of verticals up back on LI and they worked great for 40 plus an inverted L for 75 but only had 35' pines there, here the trees are a wee bit taller. The loop might have less noise than the center fed dipole but I think the gain from what I have seen on W4RNL's site is very close plus the dipole will work very well on the higher bands with a lower take off angle on some band's over the loop. I don't think that all the hype about corner fed delta loops from what I have seen holds up, the pair of verticals would spank the pants of the loop I had on 40mtrs before with the apex at 50' but from what I am reading the rectangle loop has the highest gain of all the loops to begin with but model the loop or the dipole and the dipole has about .5db more gain and slightly lower take off angle at 75" on 40 meters. I have two rolls of wire, one is the standard HD 14 and a roll of 13 copper clad stranded, I would rather just put up one or the other for now but I was wondering is anyone has ever tried these types of loops before.
Jim
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WB6BYU
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Posts: 13038




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« Reply #3 on: July 17, 2013, 09:29:09 PM »

Running the models, the loop is pretty much the same as a dipole, though as I thought
the dipole has a slight advantage for DX because of the greater average height.  If you
use an 80m doublet instead you may see about 3dB gain on DX in the favored direction,
partly due to the lower angle of radiation and partly due to the colinear gain.

For a velocity factor of 1.0, a feedline length of 34' should cover the whole 40m band
at less than 2 : 1 SWR with no other matching.  On 80m, 40' of line (again without
correction for velocity factor) will cover about 3.85 to 4 MHz.  Longer lines will move
the resonance further down the band.  The exact lengths depend on the characteristic
impedance:  450 ohms seems better than 600 ohms in this case.

So something like W5DXP's switched feedline lengths would allow coverage of both
bands without a tuner.

It does have SWR dips close to 20m, 17m, and 15m, and possibly other bands.  The
radiation pattern on 20m has 4 lobes at 15 degrees, but roughly 30 degrees from the
direction of the wire rather than broadside.


So not much to recommend it over a standard dipole on either band, which may be one
reason why it hasn't been used much.  Given the variation in feedline lengths needed for
resonance, I wouldn't be surprised if it was one of those "good ideas" that got published
without much testing.  (There are others in the literature that look good on paper but
don't hold up in actual operation or modeling.)
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KE2TR
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Posts: 130




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« Reply #4 on: July 18, 2013, 08:20:01 AM »

The loop since it is horizontal polarization may have less noise pickup then the dipole, I could also make the top and bottom wires a wee bit longer like maybe 75'-80' then the sides of the loop would be between 25' to 30' getting the baseline higher off the ground. For years I have always thought get the current point up as high as you can, have run a pair of inverted vee's up 80" year's back with the ends at 40' and they worked great but the noise factor was its week point but at that qth I had two 500' beverage's. I only have a 1/2 acre plot and if one of these loop's work  I could place another behind it and add 3/8th wave feedine to each which would make it look like a reflector and switch them back and forth with a DPDT relay. It would be a parasitic array with spacing of 1/8th wave on 80 and 1/4 wave on 40 and to the west the land drops off so it would look like 90' in that direction. The heading into EU would be 60 degrees which has always worked well for me in the past and I might be able to place the verticals for 40 out in the back for nnw/sse which would cover the times of the year were JA LP comes out of the south and there short path almost due north but for allot of the year JA on 40 comes out of the west, a kinda skewed path. Back at one of my qth's I had up a 3 element F12 40 and I have seen that many times were I would tell a guest op to point the beam due west for JA's in the morning, I miss my old station but was a single guy so had lots of antenna's back then.
 
When you were talking about feedline length were you looking at the doublet or the loop? The only thing that interests me on the loop is RX and I might be able make a parasitic array for two bands which would also help on RX even if its only 10-15 db FB that is better than none plus a parasitic two element array would lower the take off angle some what. I got a real hot day out there today, all the wire and rope is here and the shack is fully wired with good grounds on the station as well as the ameritron outside switch box, just wanna get an antenna up and get back on but want something that's outa the box per say.
Jim
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WB6BYU
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Posts: 13038




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« Reply #5 on: July 18, 2013, 08:39:03 AM »

Feedline lengths were those for the loop to give a good match to coax from there to
the shack, without any correction for velocity factor.

For the doublet you can run the parallel line back to a tuner in the shack.
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KE2TR
Member

Posts: 130




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« Reply #6 on: July 20, 2013, 10:19:01 PM »

40' works out to 3.820 1.1 swr but I didn't place the 40 mtr RG11u 1/4 wave matching xfmr for 40ty yet. I tryed a RG6 cabble but it didnt improve 40 mtr operation at all, have to use the antenna tuner for 40 mtrs. First night out I worked a bunch of local's and they all said it seemed to smoke the inverted L I had up plus a few G3's in the 75 mtr window with nice reports. I went with the loop for less local qrn pickup and I think I will be installing a seperate 40 mtr loop as well, this one seem's to hear real well so far.
Jim
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WB6BYU
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Posts: 13038




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« Reply #7 on: July 20, 2013, 10:34:30 PM »

My model said that you don't need the matching section on 40m - it showed a low
enough SWR across 40m without it when the feedline length was shortened
appropriately.

So before you bother cutting some 75 ohm line, try varying the length of the ladder
line and see how close you can get.
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KE2TR
Member

Posts: 130




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« Reply #8 on: July 22, 2013, 07:54:49 AM »

I would have to shorten the 450 ohm line way to much to raise it up  around 7.1, its at 6.2 now which is to low but 75 at 3.820 I feel I will leave alone. I did try 40 with the 75 ohm coaxial stub but it only rose to 6.4. The rectangular loop work real well on 75, I changed the feed point to the top section of the loop so that the feed line is away from the house, the XYL didn't care for that ham antenna look at the back porch look.
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