I have been experimenting with the coaxial receiving loop described in
the ARRL Antenna Book and (pages 5-19 to 5-20 17th edition) also in
ON4UN's book and also on KC2TX's web page. . Its the loop made from
20ft of RG59U and configured in diamond shape with 5ft per side. About
1 inch of braid of the coax is removed at the top of the diamond and a
resonating compression type trimmer is used at the feedpoint at the
bottom of the diamond to achieve resonance on 160M. I built the
antenna using 20ft of Belden 8241 (RG59U type cable with a capacitance
per foot of 20.5pf) . It was built in the basement and I used a junk
box trimmer and a dipped capacitor to resonant the loop. I finally
achieved resonance "indoors" by using a 390pf cap across an ARCO 463
capacitor with a range of 20 - 180 pf. Using my VOM that has a
capacitance measurement function I measure approx 480 pf across the
trimmer and the 390pf for resonance. A peak in a test signal rec'd on a
TS830 was used as the indicator. When I moved the loop outdoors the
resonance point changed and I had to slightly readjust the trimmer. So
with the loop self capacitance ( 410 pf ) and the resonating
capacitance (approx 480pf) thats a total of approx 990pf.
In the published article in the Antenna Book, reference is made to
achieving resonance using about 100pf. This reference appears to be an
error since it is not possible to achieve resonance at 160M using this
length of cable and an additional 100pf. There is another
inconsistency as well ---- figure 26 page 5-20 Antenna Book shows a
trimmer of 400pf value which is close to being correct but not quite!
In my case I was able to pass through a peak very nicely using the 390pf
fixed cap across an ARCO463 . Can anyone clarify the 100pf reference in
the ARRL Antenna book??
Receiving on the loop indoors on 160M I was hearing all kinds of crude
and weird signals and also CW and SSB. . The "bible" says that the
shield loop can be installed indoors or outdoors. In my case when I
moved the loop outside onto the deck just few feet above ground the
most of the noise and weird signals disappeared. So I am figuring that
indoors the loop was picking all the electrical noise from the stuff
running in the house. So again I have seen a different result than
that stated in the Antenna book.
Has anyone out there built one of these loops and if so can you tell me
what you have found using it. Last night I listened for a while on 160M
using my FT1000MP and didn't see very much difference between the loop
and my short "T" vertical. With the loop I am using an ARR 1-30mhz
preamp 20db gain and noise figure 2.5.
--
Frank Davis, P.Eng.
VO1HP
St.John's , Nfld.
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