"Doug" wrote in
nk.net:

By the way, the MFJ unit has been a real jewel to sweep those top of
the line VHF marine antennas with built in filters for RFI from land
mobile public safety frequencies. We have customers who have areas on
the Columbia River where police and fire departments just blast
through on a lot of the cheaper radios. The SEA 156 was the old
solution for them as it had a great receiver that was immune to the IF
image/front end over load problems. The customer does not like the SEA
157 front panel operation and we are currently testing other brands
now on a few of their tug boats. The antennas


I use a single hi-split DB Products cavity with two slugs in it as a
bandpass cavity between the VHF marine radio and the antenna. Passing the
signal through the cavity has two benefits....
1 - No paging/fire/cop/taxicab/etc. interference breaking the squelch...
2 - The radio isn't connected to the antenna, physically, and if you ground
the cavity to the ship's grounding system it keeps the lightning and static
discharges out of the marine VHF....

"Larry W4CSC" wrote
"Doug" wrote in
nk.net:

By the way, the MFJ unit has been a real jewel to sweep those top of
the line VHF marine antennas with built in filters for RFI from land
mobile public safety frequencies. We have customers who have areas on
the Columbia River where police and fire departments just blast
through on a lot of the cheaper radios. The SEA 156 was the old
solution for them as it had a great receiver that was immune to the IF
image/front end over load problems. The customer does not like the SEA
157 front panel operation and we are currently testing other brands
now on a few of their tug boats. The antennas


I use a single hi-split DB Products cavity with two slugs in it as a
bandpass cavity between the VHF marine radio and the antenna. Passing the
signal through the cavity has two benefits....
1 - No paging/fire/cop/taxicab/etc. interference breaking the squelch...
2 - The radio isn't connected to the antenna, physically, and if you
ground
the cavity to the ship's grounding system it keeps the lightning and
static
discharges out of the marine VHF....


Larry, there may be some safety benefit to that arrangement, but I'm
interested in whether your personal experience is that Decibel Product's
ho-splitter actually has some quieting results as well (besides its ?db
insertion loss ;-). As you know the grounding provides only a DC-blocker,
and unfortunately the majority of lightning's energy is not DC. In practice
most of these devices allow static-noise to pass right through, mainly
because the noise has such broad RF bandwidth to itself. Even sophisticated
lightning surge protectors that promise continual static drain, nonetheless
let most of the atmospheric noise through. With a preselector after the
static and DC-blocking devices, the noise level at least noticeably drops,
that's my experience.

Jack

"Jack Painter" wrote in
news:A1che.4837$It1.499@lakeread02:

Larry, there may be some safety benefit to that arrangement, but I'm
interested in whether your personal experience is that Decibel Product's
ho-splitter actually has some quieting results as well (besides its ?db


I don't see how a splitter, that would only drop the interference 3 or 4 dB
could reduce the intermod because it would be hitting the front end amp of
the receiver still pretty hard. With the single cavity tuned to 157 Mhz
around the middle of the marine band, the attenuation of the 152 Mhz paging
and cop bands is quite large, dropping the signal hitting the receiver to
insignificant.

As to the lightning comment, you have to see how far apart the radio's loop
and the antenna's loop is inside the metal bandpass cavity. They are on
opposite sides of the top plate and the loops go directly to ground after
making just the one turn to excite the cavity. Even the plunger rod is
directly in any path between the loops. They are, probably 9" apart,
physically, and everything is a direct ground. I doubt it would survive a
direct hit, but the radio wouldn't be the only thing destroyed in that
event. Most radios are destroyed by static discharge (St Elmo's Fire), not
direct lightning hits. The cavity idea completely eliminates those. I
didn't put the cavity in for lightning protection. I got fed up listening
to 10 paging transmitters as we sailed across the harbor.

"Larry W4CSC" wrote
"Jack Painter" wrote in
news:A1che.4837$It1.499@lakeread02:

Larry, there may be some safety benefit to that arrangement, but I'm
interested in whether your personal experience is that Decibel Product's
ho-splitter actually has some quieting results as well (besides its ?db


I don't see how a splitter, that would only drop the interference 3 or 4
dB
could reduce the intermod because it would be hitting the front end amp of
the receiver still pretty hard. With the single cavity tuned to 157 Mhz
around the middle of the marine band, the attenuation of the 152 Mhz
paging
and cop bands is quite large, dropping the signal hitting the receiver to
insignificant.

As to the lightning comment, you have to see how far apart the radio's
loop
and the antenna's loop is inside the metal bandpass cavity. They are on
opposite sides of the top plate and the loops go directly to ground after
making just the one turn to excite the cavity. Even the plunger rod is
directly in any path between the loops. They are, probably 9" apart,
physically, and everything is a direct ground. I doubt it would survive a
direct hit, but the radio wouldn't be the only thing destroyed in that
event. Most radios are destroyed by static discharge (St Elmo's Fire),
not
direct lightning hits. The cavity idea completely eliminates those. I
didn't put the cavity in for lightning protection. I got fed up listening
to 10 paging transmitters as we sailed across the harbor.

Roger that, thanks for the recommendation Larry. The pager interference is
awful in Hampton Roads, one of the many areas identified by the Coast Guard
as having serious interference to vhf marine band. The Boston and Cape Cod
areas were another area identified with that problem, I didn't know
Charleston was also so bad. At least part of the problem will be reduced
when the new narrow band radios become prevalent. I can reduce most but not
all pager interference just by setting a receiver to FM Narrow, and this
works even when active splitters, notorious for amplifying pager
interference, are used.

Jack
Va Beach

"Jack Painter" wrote in
news:S%Jhe.6226$It1.5916@lakeread02:

Roger that, thanks for the recommendation Larry. The pager
interference is awful in Hampton Roads, one of the many areas
identified by the Coast Guard as having serious interference to vhf
marine band. The Boston and Cape Cod areas were another area
identified with that problem, I didn't know Charleston was also so
bad. At least part of the problem will be reduced when the new narrow
band radios become prevalent. I can reduce most but not all pager
interference just by setting a receiver to FM Narrow, and this works
even when active splitters, notorious for amplifying pager
interference, are used.

Jack
Va Beach



As long as boaters keep demanding the cheapest piece of crap the
electronics industry can produce....and as long as boat crap discounters
keep ordering $30 VHF transceivers so they can make $100 on each
unit....the problem will never go away.....

Fortunately, paging companies are all going bankrupt from the cellphone
competition...cops are all going to UHF trunk systems...and Motorhola has
bribed the FCC so they won't be selling anyone more VHF business-band
licenses forcing them all to buy into Motorhola's overpriced trunk radio
systems...or Nextel....

On Sun, 15 May 2005 09:13:18 -0400, Larry W4CSC
wrote:

"Jack Painter" wrote in
news:A1che.4837$It1.499@lakeread02:

Larry, there may be some safety benefit to that arrangement, but I'm
interested in whether your personal experience is that Decibel Product's
ho-splitter actually has some quieting results as well (besides its ?db


I don't see how a splitter, that would only drop the interference 3 or 4 dB
could reduce the intermod because it would be hitting the front end amp of
the receiver still pretty hard. With the single cavity tuned to 157 Mhz
around the middle of the marine band, the attenuation of the 152 Mhz paging
and cop bands is quite large, dropping the signal hitting the receiver to
insignificant.

As to the lightning comment, you have to see how far apart the radio's loop
and the antenna's loop is inside the metal bandpass cavity. They are on
opposite sides of the top plate and the loops go directly to ground after
making just the one turn to excite the cavity. Even the plunger rod is
directly in any path between the loops. They are, probably 9" apart,
physically, and everything is a direct ground. I doubt it would survive a
direct hit, but the radio wouldn't be the only thing destroyed in that
event. Most radios are destroyed by static discharge (St Elmo's Fire), not
direct lightning hits. The cavity idea completely eliminates those. I
didn't put the cavity in for lightning protection. I got fed up listening
to 10 paging transmitters as we sailed across the harbor.

A 10 inch vhf cavity with .5 db insertion loss set gives about 30 db
attenuation 1 mhz away from center frequency.
At .3 mhz away it gives about 20 db loss. At only 100 khz away the
loss is around 6 db.
Can't cover much of the marine band with that. Seeing as how channel 6
and 16 are some .5 mhz apart it doesn't look like that would work too
well. This also attenuates the transmitter by the same amount.

What size cavity did you say you were using?

Regards
Gary

Gary Schafer wrote in
:

What size cavity did you say you were using?



It's about 3' long, 8" diam. Don't have any model number on it. I bought
8 at a hamfest for $15 each because it won't work on the 2 meter ham band.
Works great from Ch 10-Ch 72, about as far afield as we get....

Your cavities must be new to have such wonderful specs...(c; You could
parallel those and offset the tuning on each to cover a wider bandwidth, I
suppose....Maybe parallel two, one for Ch 6-22A and one for 68-72? Two
humps in bandpass...

On Mon, 16 May 2005 09:37:24 -0400, Larry W4CSC
wrote:

Gary Schafer wrote in
:

What size cavity did you say you were using?



It's about 3' long, 8" diam. Don't have any model number on it. I bought
8 at a hamfest for $15 each because it won't work on the 2 meter ham band.
Works great from Ch 10-Ch 72, about as far afield as we get....

Your cavities must be new to have such wonderful specs...(c; You could
parallel those and offset the tuning on each to cover a wider bandwidth, I
suppose....Maybe parallel two, one for Ch 6-22A and one for 68-72? Two
humps in bandpass...

The 8" is not quite as sharp as the 10". The 8" at .5 db insertion
loss shows: at 1 mhz away it is down about 15 db. At .5 mhz it is down
about 8 db.
At 5 mhz away it should give abut 30 db attenuation. That should wipe
out the paging transmitters but the pass band is still too narrow to
cover much of the marine band. Fine for a single frequency.

Like you say, if you had two of them in series and stagger tuned them
then you would probably be ok. You could get an adequate bandpass for
most of the marine channels. Although the insertion loss would be a
little higher. The rejection at the paging frequencies would be
greater too.

Regards
Gary

In article ,
Gary Schafer wrote:

The 8" is not quite as sharp as the 10". The 8" at .5 db insertion
loss shows: at 1 mhz away it is down about 15 db. At .5 mhz it is down
about 8 db.
At 5 mhz away it should give abut 30 db attenuation. That should wipe
out the paging transmitters but the pass band is still too narrow to
cover much of the marine band. Fine for a single frequency.

Like you say, if you had two of them in series and stagger tuned them
then you would probably be ok. You could get an adequate bandpass for
most of the marine channels. Although the insertion loss would be a
little higher. The rejection at the paging frequencies would be
greater too.

Regards
Gary

I always like the use of 1/4 wave Tuned Stubs for each Paging Frequency
in question. Seems like that always worked for me.....


Bruce in alaska
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add a 2 before @