TinkerToy Loop (LONG), tvl antenna thoughts

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From: Niel Skousen (nskousen@scientech.com)
Date: Fri Oct 13 1995 - 14:54:02 EDT


This is more of a logbook/notes dump than a how-to. I have traveled with a
dipole and Sony SW-55 occasionally for the past year (yea I know, I'm
supposed to take a Tx too !!)

Given that a hotel environment includes a significant random noise
(computers, TV's, extra video/paymovie stuff, hairdryers etc.) problem. I
have come to the conclusions that a dipole (or any form of random wire /
counterpoise) suffers from the following in a hotel environment:
- excessive noise coupling caused by folding the elements around the room,
and (Murphy's law) in the optimal position to couple to the adjacent
room's noise source.
- has to be restrung every day, after its vacuumed up....
- is never really balanced . . .
- requires a balanced tuner to work, if at all . . .

I had been toying with the loop as a balanced antenna for these reasons,
Jess's thread and wanting to try to hear the FOX got me off dead center.
The results of 1 hr of fab, and 2 hrs of tinkering is shown.

The antenna was built of 5/8in dowel, center and base pieces are hacked out
of 1in pine (approx 2 x 2) with 5/8in holes drilled to accomodate the dowel
pieces. The support arms and base are 'press-fit' and it goes together
just like the old tinker toys.... Physical fabrication was done in an
hour, the night before the plane left and cost about $2.50. 8 ft of
RF-178 (teflon/low loss version of RG-174), 14 ga Tefl.Wire, a couple of
caps, a piston trimmer, and a couple of cores were thrown in a baggie and
into the travel case. I use a 'rollaboard' style traveling case so the
maximum length was defined by the environment. Total package size is 22in
x 2 in and weighs . . 8-10 oz.

Assembly in hotel took 10 min, results are shown below for both the Gamma
match and a transformer feed. I found the balun feed to be much quieter
and yeild 6db or better SNR improvement, largely I believe to the improved
balance. The Gamma match appears to upset the balance and was noisier.

Overall this was a fun evening in the hotel, heard the fox hunt (but not
the fox) on the loop about 20 min's after I finished it.... ASCII graphics
don't do well, If there is any interest I can get a better sketch to Steve
H to post w/ Jess's loop sketch by mid next week.....

TNX for the bandwidth...
73 all
Niel

    TinkerToy Loop

           C
         / | \
        / | \
       / | \
      / | \
     |-----X-----|
      \ | /
       \ | /
        \ | /
         \ | /
           B
           |
           |
           |
       ---------

Physical data:
   22in long, 2in diameter stowed
   56in high, 44in diameter operational
   length of side: 31 in (.8 m)
   circumference: 124 in ( 3.15 m)
   Weight: 10oz
   Support: 5/8 in dowel, 22" long
                   (max.Length for rollaboard luggage)

[C] Cap:
   15pf piston trimmer + parallel Cap (see perf.notes)

[B] Balun:
   Pri 3 turn, feed w/RG-178 Coax from bottom
   Sec 1 turn, sym from top,
     grounded to coax braid at center of turn (on bottom of core)
     Core BN 63-202 (1ea, two hole balun core)

Performance:
   Loop inductance: *approx* 3.7 uh

   C= 120pf (approx: 47||47||Trim) Balun
     7.050 Mhz Tune: 6.95 - 8.30 Mhz
     VSWR 1.4, 2:1 VSWR BW: 6.950 - 7.110

   C= 120pf (approx: 47||47||Trim) Gamma - 18"
     7.106 Mhz Tune: 7.030 - 8.35 Mhz
     VSWR 1.7, 2:1 VSWR BW: 7.077 - 7.179

   C= 15pf (approx: Trimmer only) Gamma Match - 18"
     11.6 Mhz Tune: 11.6 - 24.8 Mhz
     VSWR 1.1, 2:1 VSWR BW: 11.55 - 11.82

NOTES:
- the multi-turn piston trimmer makes a much easier adjustment than a
variable with 180 deg.rotation
- measured w/ Autek RF-1 @ 8ft RG-178 / BNC

To Do/try on next trip(s):
- need a 100pf piston trimmer
- will try 2 turn, 3 turn, compare efficiency
- will try electro-static (shielded) loop
- will try coax shield as loop conductor
- when I get to it: motorize cap / tuning

----- -.. . .-- .- --... ... ... .- ------------
Niel Skousen, nskousen@scientech.com
SCIENTECH Special Projects
208-525-3742, 529-4721 (FAX) WA7SSA
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