--- Jim Dunstan <jimdunstan@...> wrote:
Hi Daniel:
Vy interesting .... I spend the winter in Toronto and the summer in Thunder
Bay ... In Thunder Bay I have a 50' tower and good antennas ... when in
Toronto I live in an apartment on the 9th flr. I have 2 balconies ....
one is the 'Ham' balcony. I operate mostly on 20M and use a pair of mobile
antennas configured as a dipole. I am interested in 40M and your loop
sounds interesting. I take it that the dimensions are about 6' in diameter
and you use RG-213 as a capacitive stub. I am interested in how you feed
the antenna. You use approximately 2.5' of coax that you move away from
the center (opposite the tuning capacitor [coax]) and find a tap with an
acceptable SWR. Tell me, do you use RG-58 for this purpose (match and
feedline) ? Also, how much RG-213 coax do you find you need to tune the
loop? How did you determine the resonance point ?? I assume moving the
feed line to different points on the loop will interact with the loop
resonance.
Thanks Jim
Hi Jim,
One of the goals for this loop was to avoid soldering (for those who are averse
to torches - for me, I'll use one, but I want to try to keep costs down). All I
used for this was a hammer, and a screwdriver... oh - and a pocket knife and
wire cutters for trimming back the coax insulation, shield, and cutting coax -
oh... and a hack saw for a small amount of cutting of the pipe ends (notched
for compression fit over dowel) and cutting a 3/8" wooden dowel ... but no
torch. The antenna analyzer I used was extremely valuable. I will eventually
post a detailed parts list if I determine that this is easily reproducible.
I used light weight RG-8X for the feedline/match and the thicker RG-8 (#213 to
be more specific) for the capacitive stub (withstands high voltages better than
RG-58/8X). I don't know at what voltage it will break down, but it seemed to
handle my 5-10W without failing. I wouldn't try 50-100W... it would probably
make for an exciting light show followed by smoke.
I don't know how efficient it really is. Theoretically - it is still only
~10-15% efficient on 40m - based on calculations from loop programs. I won't
really know how effective it is until I start comparing it with another antenna
nearby.
For the capacitance stub - I started off with enough coax on the capacitance
stub to go to the center of the loop (where the cross pieces meet - roughly 38
inches). The break in the loop and the capacitance stub are at 12 o'clock on
the loop. I used an MFJ analyzer and discovered it was 4:1 SWR around 6.0 MHz.
Then I tweaked the location of the feedline matching section with the antenna
sitting about 1-2 feet off the ground in a horizontal configuration - seated on
four plastic lawn chairs. I discovered that the best match to be had involved
using an 18" bungee cord between the center/cross-pieces and the matching coax
stub (not to be confused with the capacitance stub). This formed a type of
equilateral Y between the 6 o'clock position of the loop where the shield of
the feedline was tied to the loop, the 4 o'clock position where the end of the
feedline was clamped to the loop, and the center of the loop where the bungee
cord was attached. I did this after I discovered that when the feedline
matching stub was loosely flopping near the loop, that the SWR would vary
significantly. When I pulled it away from the loop - I seemed to get the best
SWR. I will take a couple photos and post them this evening if I get home in
time before dark.
After I got the match near 1:1 - I then started trimming back on the
capacitance stub - 2" at a time yielded roughly 200 kHz increase in frequency.
I reduced my final cut to 1" at a time as I approached 7 MHz. I was able to get
it nearly tuned for 7.0 MHz, and then took off another 1/4 - 3/8 of an inch to
get optimum resonance near 7040 for QRP use. I'd guess that the approximate
length of the RG-213 is at around 25-30 inches. I think I may have had to move
the matching stub some after moving the tuning frequency.
Finally - I moved the antenna to near my window (single story home) - rechecked
SWR (now 1.5:1 instead of the beautiful 1:1 I had previously - caused by near
field coupling with metal objects?) - adjusted matching point in hopes of
fixing SWR - and ran the feedline through the screen into the shack. I leaned
it alongside a wooden fence between my house and the neighbors (roughly 10-12
feet between homes). They have an aluminum gutter not more than 5-7 feet away
on the side of their garage. The loop is not more than a foot above the ground
at its lowest point.
I've since looked up my neighbor ham I contacted last night, and he is about
1/2 mile east of me (and my the antenna is oriented for N-S propagation ...
null to the E-W). Likewise, it was a surprise to work Arizona since it is also
towards the null. I'm guessing that there was more of an NVIS component of my
signal that was making it to Arizona early this morning than a low angle (20-40
degree) component. I don't expect to work much E-W because the two houses are
on the East and West sides of the antenna.
When I tried the antenna at lunch time, I guess the sun dried off the moisture,
or caused enough heat related expansion(?) to move resonance up about 10-20 kHz
(not critical considering that this antenna was 2:1 from 6980 to 7095 at lunch
time).
I'm guessing that the matching stub is partially magnetic coupled because the
coax shield is left in place between the 6 o'clock and 4 o'clock positions on
the loop (capacitor is at 12 o'clock). I will find out through trial and error
if I get improved matching (or end up destroying more coax) by removing the
shield connection of the matching stub this evening. Maybe I'd better stop by
the "candy store" for my RG-8x before I proceed with the experiment.
I've seen mention of the Rock-loop using a small light to help determine
resonance. Can anyone shed some light (pun intended) on what bulb size would be
appropriate, and how many turns to use to determine if the antenna is
effectively radiating and not just warming copper and earth?
Something else worth noting - I have to turn off my nearby computer (LCD
monitor - not CRT), and its UPS in order to achieve quiet conditions. This
equipment is also about 10 ft away from the loop - broadside to it (should be
in the null). But - when I turn it off, I get the quietest conditions I have
ever experienced at my location (usually I am overrun with powerline noise). It
almost feels like I've moved from inner suburbia to an exclusive rural location
(except for occasional wideband QRM from other appliances that I think are in
my neighbor's home) when I turn off my UPS and all the electronics attached to
it (running the K2 off of a car-starting SLA battery with built in charger...
the charger is another RFI generator).
So - my next endeavors include playing around with the feedline match
configuration - possibly relocating the loop to the front yard (away from RFI
in the shack) ... maybe even up 4-6 feet (1-2 m) above the ground to reduce
ground losses - and trying to prune a 30m coax stub if I get the time. I plan
on parking on 30 and 40m for most of this winter since they seem to work well
for QRP during this time of the solar cycle - and 80m is just a little big
right now for this endeavor (although... I guess it might work with about a 6
foot long capacitance stub? - just very inefficient).
That's all of my thoughts for now.
- Daniel / AA0NI
