SRF antenna transplant comments


sloshatron <wa1gwh@...>
 

Thank you for the reply, Gary. It's nice to know that only matching
up the inductance is close enough!

Thank you for the comment and question, Guy. My understanding is
that despite their being two separate coils on the stock SRF
loopstick and despite the bigger coil being tapped the entire unit
works as a single untapped inductor in the circuit. The tap does not
seem important (it may have been used in some other model?) and the
second smaller coil is slid along the bar to vary the inductance of
the whole loopstick unit for alignment purposes.

So with regard to a possible spider web transplant, I was thinking of
a single continuous wind with just a touch more total inductance.
Alignment would be accomplished by removing/adding turns or partial
turns. Since this could not be done with the radio running and while
listening to a test signal, I'm wondering if measuring the audio
output with a meter (as shown in the service manual) each trial as
turns are removed/added and the radio repeatedly turned on and off
for the procedure might work.

I'm just kind of dreaming about this. The Rap N Tap discussion group
of the Xtal Set Society has kicked around the idea that ferrite can
vary in its quality, even from the same supplier. It is indeed
easier to use, especially for this application, but I only have a big
old ferrite bar of unknown material and quality on hand. I wound on
enough #20 PVC coated hookup wire to get me into the top half of the
MW band and compared it to a 4 1/2 inch air core basket weave
solenoid coil of #21 solid using the same variable capacitor for both
tests. This was a booster bar antenna setup like Gil Stacy's idea,
my SRF was operated stock. The result was that the air core solenoid
was way better than the ferrite bar. Use of the air core coil gave
substantial signal level increase.

This was not a fair test, however, because the winding on the ferrite
bar is substantially non-optimum. I covers too much of the bar and
was not separated an eighth inch or so from the bar (based on info
from our guru, Ben Tongue).

I am planning to do a better winding on the ferrite bar when I figure
out how and get around to it. Based on my scan of Ben's article, #30
wire centered with turns spaced at least one wire diameter and about
an eighth inch off the bar should be a better configuration for this
type of inductor.

Ben's article can be found at:

http://www.bentongue.com/xtalset/29MxQFL/29MxQFL.html

Whether the performance differences he indicates are observable with
amplified radios like the SRFs remains to be seen.

Garry
near Syracuse, NY


dhsatyadhana <satya@...>
 

Hi Garry:

Neat experiment! A thought on the gain difference between the two - I
have been struck by the gain increase when I put as many turns as
possible on loops. I have a large solenoid-wound PVC box loop with a
LW coil and a MW coil, both of which can tune 540 khz with the same
variable cap. On 540 khz, the gain on the upper end of the LW coil is
dramatically larger than the lower end of the MW coil. So, perhaps if
you only had enough turns on your ferrite to get to the upper end of
the MW band, putting the "normal" amount on the ferrite will make it
more competitive with the air-core spider loop?

Kevin S
Bianbridge Island, WA

--- In ultralightdx@yahoogroups.com, "sloshatron" <wa1gwh@...> wrote:
I only have a big
old ferrite bar of unknown material and quality on hand. I wound on
enough #20 PVC coated hookup wire to get me into the top half of the
MW band and compared it to a 4 1/2 inch air core basket weave
solenoid coil of #21 solid using the same variable capacitor for both
tests.