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Author Topic: Attempted 6 meter lawn chair loop antenna  (Read 485 times)
WA2ISE
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Posts: 1174




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« on: October 11, 2018, 04:28:03 PM »

Found a lawn chair a neighbor was tossing, so I decided to make a 6m loop, like
https://www.qsl.net/ash_ares/lawn%20chair%20antenna.pdf

My chair was an inch narrower, so I made the other length an inch longer.  Used some
aluminum platters from a dead hard drive, and other misc hardware.  

It seemed to dip a little when I hooked it up to the rig (IC 756Pro).  And I found that the feedline length mattered. Which means it wasn't matching that great.  

I don't have an antenna analyzer that could do 50MHz (I do have one for HF), but I do have a grid dip meter (a mil spec one).  I checked its frequency calibration to see if my radio would hear it when I tuned it to 50.313 (pretty close).  And dipped the antenna and found it resonated around 42MHz.  Didn't think it would be that far off.  I ended up removing the cap plates, and folding one of the arms to make a smaller cross section, and creating capacitance.  


Pretty ugly.  and not suitable to be outside.  But with some styrofoam and releasable ty wraps, I did get it resonant near 50.313 though it's very twitchy.  I did avoid having it next to metallic objects in the attic, though the old books probably have some effect.  

So I might be able to do some FT8 on 6m, and of course the band is now dead.  Cheesy
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KE6EE
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« Reply #1 on: October 11, 2018, 07:51:05 PM »

Don't sit it while transmitting.

Or put some aluminum foil you know where.
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KD6RF
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« Reply #2 on: October 12, 2018, 02:25:13 AM »

I like weird antennas Smiley

If feedline length matters, it usually mean common mode current flow.  I wonder if isolated loop feed would provide less common-mode and therefore more efficiency and more consistent behavior compared to the gamma?

When this is perfected, you can try your recliner for a little gain in a "Hentenna" configuration (unless you're just chicken)  Shocked



I haven't really tried verifying the Hentenna, but here is a write up and you can decide for yourself ---> http://www.nc4fb.org/wordpress/6-meter-hentenna/



« Last Edit: October 12, 2018, 02:30:46 AM by KD6RF » Logged

VTenn Antennas
Bay Area Technical Equip Rental and Test Range
http://vtenn.com/Blog/
WA7PRC
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« Reply #3 on: October 12, 2018, 02:41:59 AM »

If you open the loop, you'll have a circular dipole. If you then rotate the legs 90°,
you'll simultaneously have horizontal and vertical polarization like many FM broadcast
transmit stations have, similar to this:

This one is Gamma Matched but, it can be driven using other means. You can stack
'em for more gain. IIRC, the FM BC transmitter I worked on in the 70s drove 6 bays
w/ 16KW for 100KW ERP (70KW/H + 30KW/V).  Grin
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WA2ISE
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« Reply #4 on: October 13, 2018, 04:53:03 PM »

UPDATE

I used one of the upper portions of the lawn chair to create a smaller loop, to get the resonant frequency higher.  And used a ceramic trim cap to tune it.  Used a grid dip meter to find the resonant frequency and to adjust it to 50.313MHz. This kind of trim cap would not stand the weather, though.   A little less ugly.  The gamma match isn't that great a match, but my rig's tuner can deal with it.  
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WA2ISE
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« Reply #5 on: October 13, 2018, 07:04:56 PM »

I can never get gamma matches to work right.  It seems to be black magic....   Sad

I did get the loop to resonate at the desired frequency. 
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WA7PRC
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« Reply #6 on: October 13, 2018, 10:56:13 PM »

I can never get gamma matches to work right.  It seems to be black magic....   Sad

I did get the loop to resonate at the desired frequency. 
My first was as a Novice ca 1970, applied to a Gotham 15m yagi. After working thru the math, it worked the first time. I've also made it work on a homebrew 2m yagi, without a lick of trouble.
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KM1H
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« Reply #7 on: October 14, 2018, 05:25:45 PM »

In 1956 I was given a 2 el 20M Gotham yagi that was storm damaged and converted it into a 3 el for 15 using existing parts plus scrounging at a local scrap metal dealer and the ARRL HB.

The Gamma match was copied from a CQ Magazine article and worked fine with a surplus variable cap in a box. In later years at 1200W I copied/modified the Cushcraft method which was to strip the jacket and braid off RG-8/213 and stuff it down an aluminum tube. Adjusting the shorting strap for a near perfect match was rather simple with the yagi semi vertical and the reflector about 6' off the ground sitting on an old wooden stepladder. The boom was supported with a rope over a tree branch.

The longest antenna I did this with was a 4 el HB 20M yagi on a 40' boom and highest band 8 el on 6M on a 35' boom which used an all metal T Match and again copied from Cushcraft.

Long boom yagis to 432 MHz were also tuned semi vertically using T Matches on 144 and 222 MHz.

KISS

Carl
Ham since 1955
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WA7PRC
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« Reply #8 on: October 14, 2018, 08:56:22 PM »

In the late 70s, a buddy gave me a 3-el 11m yagi. I took it home and a coupla hours later, I had 6-el on 2m w/ Gamma Match. I stood it up on a pole next to a 6' step ladder, and in about 10 min, tweaked the Gamma match so my IC-22S output meter indicated maximum. The rig has a VSWR bridge built-in for protection but displays only relative forward power. Max fwd = low VSWR.  Smiley

That thing w/ only 10W from my FM squawk box driving it worked pretty darned good. I was talking all over the place from my crappy VHF location.  Smiley
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WA2ISE
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« Reply #9 on: October 15, 2018, 03:38:25 PM »

I decided to just go with a simple dipole, and wound a choke with the feedline (7 turns around a discarded water bottle, the kind millennials bring to meetings at work).  I melted some holes in the bottle to help hold the coil together, along with some packing tape. It's not out in the weather.  The wire was some 12 gauge house wire stapled to the roof rafters in the attic.  

My rig is happy, Cheesy this antenna presented an SWR around 2.5:1 or so, which the rig's tuner could deal with.   The feedline is about 25 feet of RG58, which should only lose me 3/4dB at 50MHz.  Cheesy I didn't do any fine tuning of this dipole.   The other wires you see run mostly perpendicular to the dipole, so they should have minimal effect.


There's something I've missed with gamma matches....  
« Last Edit: October 15, 2018, 03:43:21 PM by WA2ISE » Logged
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