"Lostgallifreyan" wrote in message
. ..
"amdx" wrote in
:

Well most tuner/antenna matcher are designed to take the impedance that
the
antenna cable presents and transformer it to 50 ohms. Your problem is the
Sangean is probably not 50 ohms.

Ah, but in reverse? I don't know if this is good thinking, but if the
coax
IS 50 ohms, could I not put the 50 ohm connection onto the coax and tweak
the
other end (on the input instead of the coax) impedance by watching the
signal
strength meter on the receiver?


Ideally you would adjust the impedance of the antenna to match 50 ohms
then attach your 50 ohm coax, then connect to a 50 ohm receiver. (you don't
have)
But trying to adjust an antenna that is way out there (say 4000 ohms with
200 Reactance)
from the shack is not an easy task.


(Might be undeed according to posts people made earlier that suggest that
losses here might matter less than what happens on the longwire end of the
coax).

I'm in agreement with those that say just put up a wire and listen.
Then pick your favorite band and put up a resonant antenna and compare the
two.
Then your learning something.
Mike

"amdx" wrote in
:

(Might be undeed according to posts people made earlier that suggest
that losses here might matter less than what happens on the longwire
end of the coax).

I'm in agreement with those that say just put up a wire and listen.
Then pick your favorite band and put up a resonant antenna and compare
the two.
Then your learning something.
Mike


Right now I have no idea what my favourite band might be. I'll try the
basic wire, I used to try that many years ago as a kid, in places where it
was easy, on private land with space for it. But that was the seventies, in
the country. Now I'm in an inner city full of mobile phones and computers,
things have changed so much that I know it won't be the same. I like that 18'
whip scheme you linked to though, I really like that one and want to try it.

In article ,
Lostgallifreyan wrote:

Right now I have no idea what my favourite band might be. I'll try the
basic wire, I used to try that many years ago as a kid, in places where it
was easy, on private land with space for it. But that was the seventies, in
the country. Now I'm in an inner city full of mobile phones and computers,
things have changed so much that I know it won't be the same. I like that 18'
whip scheme you linked to though, I really like that one and want to try it.

Lost-

I agree that the "basic wire" antenna approach will almost certainly
work. At least you will get your feet wet. That is the kind of
external antenna most often used with that type of radio.

If you want to continue the search for input impedance, consider trying
something like an MFJ Antenna Analyzer (MFJ-259 or 269?), which uses a
sufficiently low signal level that it shouldn't hurt the radio. Instead
of analyzing the antenna, use it to analyze the input of the radio. You
can use the information to design a matching circuit, but you may find
that the improvement is disappointing for reasons already discussed.

Fred
K4DII

Fred McKenzie wrote in
:

In article ,
Lostgallifreyan wrote:

Right now I have no idea what my favourite band might be. I'll try
the basic wire, I used to try that many years ago as a kid, in places
where it was easy, on private land with space for it. But that was the
seventies, in the country. Now I'm in an inner city full of mobile
phones and computers, things have changed so much that I know it won't
be the same. I like that 18' whip scheme you linked to though, I really
like that one and want to try it.

Lost-

I agree that the "basic wire" antenna approach will almost certainly
work. At least you will get your feet wet. That is the kind of
external antenna most often used with that type of radio.

If you want to continue the search for input impedance, consider trying
something like an MFJ Antenna Analyzer (MFJ-259 or 269?), which uses a
sufficiently low signal level that it shouldn't hurt the radio. Instead
of analyzing the antenna, use it to analyze the input of the radio. You
can use the information to design a matching circuit, but you may find
that the improvement is disappointing for reasons already discussed.

Fred
K4DII


I'll pass. I think the reason no-one knows is that as you (and others)
say, it's not important enough. What does seem important is to try to reduce
localised noise, and to break the current link to protect the radio input
from static discharges. Whether I use coax or a balanced loop made from
speaker wire, it loooks like my next step is to get Amidon FT-114-75 ferrite
cores to play with, and in Britain I can't easily do that, but if anyone
knows a local direct equivalent to them I can try that. I'll Google for
things that fit the description (AL about 3000, permeability u=5000, about
1.14 inch outside diameter) but I think it's wise to ask here to try to save
time.

In article ,
Lostgallifreyan wrote:

I'll pass. I think the reason no-one knows is that as you (and others)
say, it's not important enough. What does seem important is to try to reduce
localised noise, and to break the current link to protect the radio input
from static discharges. Whether I use coax or a balanced loop made from
speaker wire, it loooks like my next step is to get Amidon FT-114-75 ferrite
cores to play with, and in Britain I can't easily do that, but if anyone
knows a local direct equivalent to them I can try that. I'll Google for
things that fit the description (AL about 3000, permeability u=5000, about
1.14 inch outside diameter) but I think it's wise to ask here to try to save
time.

You can probably at least start your experimentation using the common
sort of interference-suppression ferrites that are found in many
computer accessories - e.g. molded onto DC cords, VGA cables, USB
cables, and so forth. Here in the U.S. these are easily available at
electronics surplus stores, ham-fest flea markets, and other such
sources.

In my experience, these tend to be a ferrite mix such as type 43,
which is optimized for use at somewhat higher frequencies than
HF/SWL... so they will probably not be optimal for your purposes.
However, they can be made to work.

A few years ago I constructed a common-mode feedline choke for our
local ARES/RACES ham station, to try to keep 40- and 80-meter signals
from being carried back down the outside of the coax and into the
building (our signal was audible on phones in the city's "911"
emergency dispatch center... *not* good). I took several tubular
computer-interference-suppression ferrite cores (large inner
diameter), glued them end-to-end with cyanoacrylate, let them dry, and
then ran some RG-8X coax through the center and back around the
outside and through the center again. The coax looped through the
tube of ferrites three times.

This resulted in an extremely effective common-mode choke. According
to my MFJ antenna analyzer, the impedance looking up through the coax
in the usual way (standard hookup, into a 50-ohm dummy load) was 50
ohms... the ferrites had no effect at all on the differential-mode
signal in the coax.

But, when I measured the impedance along the braid (i.e. from the
ground shell at one end of the coax, to the ground shell at the other...
a DC short circuit), I couldn't get a reading at any frequency... the
meter just said " 1500 ohms". Even at the lowest frequency of
interest, these non-optimal ferrites added so much inductance to the
common-mode signal path that they were blocking the feedline current
flow very effectively.

[Unfortunately, we determined that the phone interference was caused by
direct RF pickup by the phone wiring, which was in the "near field" of
the antenna above the roof. It occurred even if we completely
disconnected the building feedline, and fed the antenna directly from
a radio located up on the roof. The feedline choke couldn't help us.]

In your situation, I'd guess that you could probably make an efficient
feedline choke by using almost any surplus ferrite toroid which is
sufficiently large to wind your feedline (coax or speaker wire)
through it a few times. Or, use several surplus ferrite cores,
end-to-end, and if they're large enough in diameter, loop the feedline
down through the center more than once.

It won't be perfect (nor as good as if you used a ferrite optimized
for use at lower frequencies) but it will probably help matters, and
will give you some sense as to whether it makes sense to go to the
trouble and expense of buying ferrites that are better for your purpose.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

(Dave Platt) wrote in
:

In article ,
Lostgallifreyan wrote:

I'll pass. I think the reason no-one knows is that as you (and
others) say, it's not important enough. What does seem important is to
try to reduce localised noise, and to break the current link to protect
the radio input from static discharges. Whether I use coax or a balanced
loop made from speaker wire, it loooks like my next step is to get
Amidon FT-114-75 ferrite cores to play with, and in Britain I can't
easily do that, but if anyone knows a local direct equivalent to them I
can try that. I'll Google for things that fit the description (AL about
3000, permeability u=5000, about 1.14 inch outside diameter) but I think
it's wise to ask here to try to save time.

You can probably at least start your experimentation using the common
sort of interference-suppression ferrites that are found in many
computer accessories - e.g. molded onto DC cords, VGA cables, USB
cables, and so forth. Here in the U.S. these are easily available at
electronics surplus stores, ham-fest flea markets, and other such
sources.

In my experience, these tend to be a ferrite mix such as type 43,
which is optimized for use at somewhat higher frequencies than
HF/SWL... so they will probably not be optimal for your purposes.
However, they can be made to work.


A complete ring is better, air gaps might make it harder to assess how good
it is unless you know enough (or have equipment) to make comparisons. I
don't. But I did find an appropriately sized type 43 ferrite ring on eBay
and put it on my watch list a few hours ago, so I guess this is what I'll buy
if I don't find better quickly. The seller has a few, I think.

A few years ago I constructed a common-mode feedline choke for our
local ARES/RACES ham station, to try to keep 40- and 80-meter signals
from being carried back down the outside of the coax and into the
building (our signal was audible on phones in the city's "911"
emergency dispatch center... *not* good). I took several tubular
computer-interference-suppression ferrite cores (large inner
diameter), glued them end-to-end with cyanoacrylate, let them dry, and
then ran some RG-8X coax through the center and back around the
outside and through the center again. The coax looped through the
tube of ferrites three times.

This resulted in an extremely effective common-mode choke. According
to my MFJ antenna analyzer, the impedance looking up through the coax
in the usual way (standard hookup, into a 50-ohm dummy load) was 50
ohms... the ferrites had no effect at all on the differential-mode
signal in the coax.

But, when I measured the impedance along the braid (i.e. from the
ground shell at one end of the coax, to the ground shell at the other...
a DC short circuit), I couldn't get a reading at any frequency... the
meter just said " 1500 ohms". Even at the lowest frequency of
interest, these non-optimal ferrites added so much inductance to the
common-mode signal path that they were blocking the feedline current
flow very effectively.

[Unfortunately, we determined that the phone interference was caused by
direct RF pickup by the phone wiring, which was in the "near field" of
the antenna above the roof. It occurred even if we completely
disconnected the building feedline, and fed the antenna directly from
a radio located up on the roof. The feedline choke couldn't help us.]

In your situation, I'd guess that you could probably make an efficient
feedline choke by using almost any surplus ferrite toroid which is
sufficiently large to wind your feedline (coax or speaker wire)
through it a few times. Or, use several surplus ferrite cores,
end-to-end, and if they're large enough in diameter, loop the feedline
down through the center more than once.


I hadn't thought of this for common mode noise suppression but I will try it,
sounds like a good idea. The rings I'm after are for the coupling at each end
of the line though, 9:1 or 10:1 at far end, 1:1 at receiver end. But I guess
that the clamp-on ferrite slug could be useful on the balanced line loop
that runs between the rings. I've also wondered if that balanced line might
better be a twisted pair with each twist perhaps 10 cm or so long to be sure
that common mode really is common, but I have no idea if that's wise or
required. Haven't seen anything to suggest I need to do it.

It won't be perfect (nor as good as if you used a ferrite optimized
for use at lower frequencies) but it will probably help matters, and
will give you some sense as to whether it makes sense to go to the
trouble and expense of buying ferrites that are better for your purpose.


80 turns.. plus another 8 for the secondary.. I read they should be close
turns, and the ring size was probably specified mainly to accomodate them
all. I will try other ferrites and carbonyl iron power type cores and
whatever else comes my way perhaps, I found four small toroids in a broken
computer SMPU that might be fun to try. Ultimately though, I'm aiming for
best shots because it's like plotting an unknown graph curve with only a few
samples, so the better the accuracy of each move, the better I can understand
any discrepancy in any one of those moves.

I have a basic philosophy that is similar to Jeff Liebermann's 'Learn By
Destroying' thing, as I also used to take stuff apart as a kid, and destroyed
plenty, but actually apart from seeing what sort of build quality was
acceptable and how discrete components were built, I didn't learn much
beyond tool handling, the thing that taught me most was when I took apart
something I had to put together again for fear of punishment if I failed and
got found out. So the closer I get to starting with something like an ideal
working part or whole, the faster I can figure out the meaning of significant
excursions from that ideal. It's kind of like the argument of whether a kid
should get his first bike new and working, or self-built from bits. I think
the first way is right because it's the fastest way to know what working
(and safety) really is, then start changing things, knowing what path was
taken and how to get back without help. Works great with computers too...