From: gene@uicsl.csl.uiuc.edu (Gene Gardner)
Newsgroups: rec.radio.amateur.misc
Subject: COMPACT 160-80-40-20M ANTENNA
Date: 27 Jan 1993 01:41:01 GMT
Organization: Center for Reliable and High-Performance Computing, University of Illinois at Urbana-Champaign
Distribution: rec
NNTP-Posting-Host: uicsl.csl.uiuc.edu
Summary: Practical construction hints for Hi-Q Loops
Keywords: Hi-Q Loops



 Someone requested a compact 160M antenna. I can report that after several 
months of experimenting with Hi-Q loops, they have proved to be surprisingly
effective.  For a city-dweller with very little space, I can verify that a 
160-80-40M Hi-Q loop is quite satisfactory, both structurally and asthetic-
ally(?)         It requires no                                
                grounds or radials. It is constructed of 1/2" aluminum tubing 
(actually surplus hard-line coax purchased as scrap-aluminum). It is vertically
  polarized with the turns separated by 20". The first 4 turns are 7.5' diameter and tune 80M  with
    a rather small amount of capacitance. The next 2 turns are separate and
are reduced to 6.75' diameter to tune 40M...again rather small capacitance
required. For 160M the 4 turns are combined with the 2 for a total of 6 turns.
The same wide-spaced split stator tuning capacitor is used in all three cases
with about 100pF max being required to tune 160M. Of course any antenna which 
is small in terms of wave-length MUST be Hi-Q to work, and for general oper-
ation this will dictate remote-tuning on the capacitor.  The Q becomes very
high on 160M (2:1 SWR bandwidth of about 8 KHz) and accordingly, the voltages
are quite high. The conventional capacitor mentioned above will tolerate only
about 200 watts before arcing (700W OK on 40-80M). It is used with the two sections in series so
that no current passes thru the rotor bearings, and the rotor is left floating.
It will be at approximately virtual ground potential). To operate with high
power it is appallingly difficult to accommodate such high voltage. A final
solution for me (also low-cost) was to improvise a concentric capacitance 
using a 2' length of 6" diameter galvanized furnace-pipe, with a 4" diameter
pipe inside...providing a 1" separation. If you can believe it, even this
arced over until I tack-soldered #14 copper "corona rings" to the
sharp edges of the pipes. The inside pipe is secured to the boom.
 The outside 6" pipe is
supported by acryllic plastic spacers such that it can be
tuned by sliding back and forth via a 1' length of threaded 1/4" nylon rod 
lead-screw (or you might use conventional metal lead-screw with 1' insulated 
extension).  The entire array is supported on three 2" I.D. PVC pipe, 8.5'
long....parallel to the earth and mounted to a 4'x 4' wooden post. The top
boom is about 12' high and supports the top of all 6 turns in 1/2" slots.
The bottom-most boom supports the bottom of the 7.5 diameter (inner turns) and
extends  to support the "furnace-pipe" capacitor. The 3rd boom supports the
outside ends of the 4 turns, the capacitor-motor-assembly, and extends to 
support the bottoms of the 6.75' diameter 2 turns. The Hi-Q loop also is an
effective "filter" for all frequencies except near resonance, and is a some-
what "quieter" antenna for some types of noise.
   It works so well on the 160-80-40M band that I couldn't resist a 2-turn
35" diameter version on 20M....now turned parallel to the earth for hori-
zontal polarization and put atop a 40' pipe(5/8 wavelength)   ...pre-tuned to a 
favorite small frequency band
(2:1 SWR about 40 KHz). Again, to withstand 700W,I had to use
almost 1" gap with concentric shells (part of 2 lb coffe can on outside, 
and smaller tin-can on the inside.  However, at this higher frequency, the
acryllic plastic softened and Teflon 1" spacers were required.
  On all bands, the response is much better than was ever expected...comparing
favorably to some of the "bigger signals" on the band using 700W. The loops are
very easy to match to 50 ohms by simply attaching the shield of the coax to the
electrical center of the loop and extending the center conductor experiment-
ally to find a near-perfect match. It might be considered "1/2 of a Delta"
match.
   Many of my contacts have urged that I try to get this information published
and I have reason to believe that it might appear in the APRIL issue of
WORLD RADIO...published in California.  If so, it will contain photographs and
and I have reason to believe that it will appear in APRIL issue of WORLD RADIO
published in California. If so, it will have photographs and detailed   
sketches.                   73 from W9RWZ


:wq

detailed sketches.              73 from W9RWZ
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