A while back I had mentioned that an air core coil had greatly
outperformed a chunky surplus ferrite bar antenna on my SRF-49 when
used as an external booster antenna. I had quickly wound the ferrite
with #20 PVC hookup wire which was centered and occupied just over
half the total length.
I have now rewound the ferrite bar with #24 plastic covered wire
taken from 4 conductor telephone cable. (The insulation looks and
feels like polyethylene, which is great as a dielectric if so.)
There is a single wrap of flexible foamed polyethylene packing sheet
about 1-2 mm thick between the bar and the winding. I started with
the center 1/3 of the bar close-wound with the #24. I then unwrapped
until the inductance was about the same as my air core coil.
I used the same variable capacitor for listening tests on stations at
1010 kHz and 1590 kHz at mid-day for steady sigs. The coils were
axially aligned with the bar on the SRF-49, although the actual axes
were about 2" above the radio bar because the diameter of the air
core is about 4" and I set it on the pine board where the radio is
fastened. I kept both test coil axes at the same height.
Each coil+cap combo was moved away from close to the radio until the
sig strength just started to drop off (as best I could tell by ear
and with AGC on the radio). Four or 5 comparisons were made for each
The ferrite bar was much better this time, although not as good as
the air core coil. The air core coil was more noticably better at
1590 than at 1010. In each case, only occasional words were copiable
with the boosters moved way away from the SRF-49. Comfortable 100%
copy was obtained with each booster at both frequencies, although
there was still background hiss. (In fact, I started to listen to a
dumb talk show!)
So, it would appear that an air core coil with a "loop" aspect ratio
(large diameter, short length) may still be an excellent alternative
to a ferrite bar coil. And, it looks like my chunky old ferrite bar
might be a good candidate for an SRF-49 transplant antenna if I
decide to go that route. But I'll have to wind my own coils.
Basket wound coil of #21 solid with insulating film:
4.1"D by 1.2"L, 41 turns total about 44.0 feet.
Ferrite bar coil of #24 plastic insulated telephone wire close wound
over poly sheet (1-2 mm):
bar: 123mm L x 31mm W x 11mm thick
coil: 35mm long, centered, about 10.9 feet of wire (a lot less than
the air coil, wire not much smaller but close wound with unknown
I hope to soon have some photos of my setup.
near Syracuse, NY