I'm trying to come up with some ideas for multi-op field day station
isolation. Main problem will be front end desens between Voice and CW
portion of the same band. Main concern is on 20M

I'm thinking simple stub filters. Though they are wide, I would think
that there would be a few DB of isolation 100Khz away. Perhaps several in
parallel to narrow it up a bit.

Would a simple LC resonant circuit with perhaps 20-30Khz of BW work?

Any thing else I should be considering?


Thanks You!

On May 6, 11:29 pm, No Spam wrote:
I'm trying to come up with some ideas for multi-op field day station
isolation. Main problem will be front end desens between Voice and CW
portion of the same band. Main concern is on 20M

I'm thinking simple stub filters. Though they are wide, I would think
that there would be a few DB of isolation 100Khz away. Perhaps several in
parallel to narrow it up a bit.

Would a simple LC resonant circuit with perhaps 20-30Khz of BW work?

Any thing else I should be considering?

Thanks You!

In general, you can make a better filter in a given volume by using
lumped LC filters at HF frequencies, than by using transmission line
stubs. The reverse is true when you get into the hundreds of MHz
region.

You need high Q elements to get good isolation and not incur too much
loss when frequencies are spaced that closely. A saving grace is that
you probably don't need a whole lot of attenuation, assuming the
receiver front end you're trying to protect isn't too bad. It's not
like you're trying to completely kill the offending signal, just get
it down to where it doesn't cause trouble. Intermod (third order
distortion) generally drops around 3dB for every dB you drop the
signals causing it. Desense should similarly go away fairly rapidly
as you drop the big signal levels.

Looks to me like a design with five coils and five capacitors and what
at least I consider practical values should give you 25dB differential
between the passband and the stopband, with a 100kHz separation
between the two, and a loss of about 5dB in the passband if you use
Q=500 coils. Air core coils about an inch and a half in diameter
should give you Q that high. I haven't tried optimizing the design,
and may be able to do a bit better than that with the same number of
parts. I can imagine building it "on the cheap" in a string of tin
cans soldered together, or else by using pieces of copper-clad
soldered together. (I've built several somewhat similar filters using
copper clad for the shielding, and they work quite well.) Would a
suggested design be helpful?

Another option (or something to consider in addition) is a small loop
receiving antenna that you can orient to null the other transmitter.
I believe N6RK just gave a paper on doing this, though with emphasis
on lower bands. He may have a PDF of the paper he'd be willing to
share.

Cheers,
Tom

On Wed, 07 May 2008 09:59:33 -0700, K7ITM wrote:

On May 6, 11:29 pm, No Spam wrote:
I'm trying to come up with some ideas for multi-op field day station
snip

These are some great things to think about.

Perhaps I may have been hasting into concluding we needed the filters in
the first place when some more easily implemented practices should be
tried first.

I think the best thing will be to only use the "good" radios on the same
bands and better utilize spacing and polarization to see what best we can
achieve. One of the other ideas I think we will be considering will be to
limit power for CW and digital modes as well. I am also having high hopes
of separation as I think there may have been some coupling induced by the
common ground and shared power supplies. Perhaps having two compounds
with separate infrastructures and space would reduce problems to a more
tolerable level.

Thank you all for you response!

No Spam wrote:
On Wed, 07 May 2008 09:59:33 -0700, K7ITM wrote:

On May 6, 11:29 pm, No Spam wrote:
I'm trying to come up with some ideas for multi-op field day station
snip

These are some great things to think about.

Perhaps I may have been hasting into concluding we needed the filters in
the first place when some more easily implemented practices should be
tried first.

I think the best thing will be to only use the "good" radios on the same
bands and better utilize spacing and polarization to see what best we can
achieve. One of the other ideas I think we will be considering will be to
limit power for CW and digital modes as well. I am also having high hopes
of separation as I think there may have been some coupling induced by the
common ground and shared power supplies. Perhaps having two compounds
with separate infrastructures and space would reduce problems to a more
tolerable level.

Thank you all for you response!

Don't neglect common mode feedline current as a potential source of
unwanted coupling. All your efforts to control polarization and
physically separate antennas can be largely wasted if your feedlines are
radiating.

Roy Lewallen, W7EL

K7ITM wrote:
...(I've built several somewhat similar filters using
copper clad for the shielding, and they work quite well.)
Would a suggested design be helpful?..

I would like to hear of your designs, please.
Particularly how you physically work the
copper fabrication/solder etc.

Thanks -

Craig 'Lumpy' Lemke

www.n0eq.com

On May 8, 3:03 pm, "Lumpy" wrote:
K7ITM wrote:
...(I've built several somewhat similar filters using
copper clad for the shielding, and they work quite well.)
Would a suggested design be helpful?..

I would like to hear of your designs, please.
Particularly how you physically work the
copper fabrication/solder etc.

Thanks -

Craig 'Lumpy' Lemke

www.n0eq.com

Hi Lumpy,

I'm fortunate to have access to a shear that cuts fiberglass/epoxy
board stock reasonably cleanly. I use all double-sided stock, copper-
clad on both sides. I typically set up the stop for, say, 2 inches,
and make a bunch of square pieces. They'll become end pieces and
partitions between "cells". Then I cut a couple 2 inch wide strips
using the same setup; these will be the sides, and they are whatever
length I need. The base piece is typically a quarter inch wider (or a
bit more) and the same length as the sides. I plan out where I need
holes, and punch them in the partitions and end pieces (or sides, if
that's what I want). Then I tack one of the square pieces just short
of the end of the base, so I can solder it to the base on both sides.
Same on the other end. Then side pieces go in, tack-soldered to the
base and the two end pieces. Avoid tacking where partitions will go.
Put in partitions where you want, and tack them. Then run a bead of
solder along each place where two pieces of copper-clad come
together. Occasionally I'll also put in something to support coils or
wires, too. Then I built the filter...soldering some parts to the
copper clad for ground.

Some filters I can get by with a minimum of partitions. I would
expect for a sharp cutoff filter like the "FD adjacent band" filter
that I'd need to use a partition to separate any two adjacent
resonators.

When I first started making filters this way, I was expecting to have
to seal off the open end of each cavity. The description above only
puts copper-clad on five of the six sides of each cell. I was amazed
to find that it's practically never necessary to do that. For
example, I have a 1MHz bandpass filter that has stops specifically at
2MHz and 3MHz. I used it to clean up the output of a signal generator
so I can test for harmonic distortion. That filter shows about 120dB
attenuation at 2 and 3 MHz, in spite of the open-topped cells.

If a picture would help, I'd be happy to send one via email.

Cheers,
Tom

K7ITM wrote:

I'm fortunate to have access to a shear that cuts fiberglass/epoxy
board stock reasonably cleanly. . .

For those of us who don't, I've found that tin snips do a good job and
without the irritating dust you get if you saw it.

Roy Lewallen, W7EL

Roy Lewallen wrote:
K7ITM wrote:

I'm fortunate to have access to a shear that cuts fiberglass/epoxy
board stock reasonably cleanly. . .

For those of us who don't, I've found that tin snips do a good job and
without the irritating dust you get if you saw it.

Roy Lewallen, W7EL

I've also used a table saw with a VERY fine-pitch blade, set just above the
table. It makes nice 90° corners. A Shop-Vac keeps the dust from getting
out of hand.

73,
Bryan WA7PRC

K7ITM wrote:
...(I've built several somewhat similar filters using
copper clad for the shielding, and they work quite well.)
Would a suggested design be helpful?..

If a picture would help, I'd be happy to send one via email.

I'd like to see pics please.
My return is valid.

Thanks mucho


Craig 'Lumpy' Lemke

www.n0eq.com

On May 8, 9:56 pm, "Lumpy" wrote:
K7ITM wrote:
...(I've built several somewhat similar filters using
copper clad for the shielding, and they work quite well.)
Would a suggested design be helpful?..
If a picture would help, I'd be happy to send one via email.

I'd like to see pics please.
My return is valid.

Thanks mucho

Craig 'Lumpy' Lemke

www.n0eq.com

I've posted some pix at http://mysite.verizon.net/bruhnstv/. They'll
be there for a short time.

Cheers,
Tom