I made my second attempt at a compact transmitting loop (single turn -
40m) last night. It is 20 ft of soft (flexible) 1/2 inch copper tubing
on a wood frame. I used a piece of RG-8 (213) for a capacitance stub,
and have it fed using a type of match (which I'm wondering how to
classify). I removed about 1 inch of outer insulation of my coax from
the rig about 2-3 feet from the end, attach it at the mid point of the
tubing (compressing the shield of the coax against the loop with a
hose clamp... but not crushing the coax), and then expose the center
conductor at the end of the feed line and attached it where I got a
minimum SWR (moved it around with a hose clamp for best SWR).
I'm in Oklahoma City. My first contact was with a neighbor (< 1 mile
away). This was our first contact ever, and at 11 PM local time on
40m, he was the strongest station on the band. He was full scale (with
my Elecraft K2's attenuator on) using 2 W into his inverted-v up
30ft, and he reported me as 599 (don't know how strong I really was).
My second contact this morning was with a station in Mesa, AZ. The
band was just starting to open up as the sun started to light up the
morning sky. Initially I was 429 at 7 AM, then 559, then 579 as we
finished up at 7:30 AM. We were both running 5W.
Here's my concern. The loop is not 'compact' when compared with MFJ's
model. When I calculate what my bandwidth should be using the various
loop programs, it shouldn't be much more than 10-20 kHz for 2:1. All
of my connections are using hose clamps with the connections spread
along the surface of the copper tubing (no soldering - but enough
surface area contact to minimize resistive losses). My 2:1 SWR
bandwidth is 100-120 kHz!
Is this due to the match I am using? Is there too much resistance in
my connections? Am I experiencing this same phenomenon like I did last
time with my 1/4" 20ft loop on 20m using #10 insulated wire inserted
into the loop for capacitance (make shift - coax) because the copper
tubing is not as low in resistance as it should be? Since this is
flexible copper - maybe it is an alloy that is not as conductive as
normal copper ... thus affecting my Q?
My next variation will be to cut off the shield of the coax just past
where it joins the loop at the midpoint. It may be that my match is
acting less as a match, and more as a loaded 2 ft antenna? Maybe this
will result in a much higher Q (and improved efficiency?).
I will try to do some side-by-side testing between the loop and my
buddipole antenna to compare noise levels as well as signal reports
from other stations to determine which antenna is more effective at
different distances (local vs long-range).
Your thoughts and ideas would be appreciated.
If there's more info you need on the construction and design, let me
know. If you live within 200-800 miles of OKC and want to try a
contact on 40m some evening, let me know.
Thanks,
Daniel / AA0NI
Oklahoma City
