On May 30, 12:50 pm, scooterspal wrote:
OK. I have tested the frequency and found it to be 75.489Mhz. Should
have done this first, sorry for the confusion. Comtek did make systems
that operated betweeh 72 and 76Mhz. This is one of those.

That said, where do I stand antenna wise?

Thanks!

Depending on how large a distance you want it to cover, you may get by
with a considerably shorter antenna than the 48 inch recommended
size. But you also didn't say, that I could see, that there was a
problem using a 48" piece of wire. Obviously it's not too critical
since in use the mic cable can be bent in all sorts of different
shapes and it still operates. In other words, try to not make this
more difficult than it needs to be. If you need the mechanical
antenna to be shorter than 48 inches, just wind that length of wire in
a spiral, spaced out as much as you can accommodate. Try that and see
how it works. Easy enough? If you need higher performance, that's
possible, but in general very short antennas won't be as efficient as
straight antennas long enough to be self-resonant at the operating
frequency. If you need good performance from a short antenna, get a
system that operates at a higher frequency.

Cheers,
Tom

On Fri, 30 May 2008 11:36:25 -0400, scooterspal
wrote:

Hello and thanks for ahy help you can provide.

I have a small assisted hearing type transmitter that normally uses the
shield of the mic cable for the antenna. I need to change to a fixed
antenna for a particular application where there is no mic cable to
extend out.

The manufacturer says the antenna wire for this 651.85Mhz unit should be
48" long.

Can anyone tell me what that would equal in a rubber ducky antenna?

Thanks.

Which model do you have?
http://www.comtek.com/sitemap.html
choose the model, and the device frequency range will be listed. They
only show three frequency ranges, 216-217 Mhz, 72-76 MHz, and 76-78
MHz. They also have several options for external antennas, but I
think these are primarily for the base transmitters.

As mentioned in an earlier reply, if you build your own antenna, the
device _may_ become compliant.


--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

K7ITM wrote:

If you need the mechanical
antenna to be shorter than 48 inches, just wind that length of wire in
a spiral, spaced out as much as you can accommodate. Try that and see
how it works. Easy enough? If you need higher performance, that's
possible, but in general very short antennas won't be as efficient as
straight antennas long enough to be self-resonant at the operating
frequency...

Is that all a rubber ducky antenna is? 48" of wire wound around a
flexible core of some type?

If so, what do I look for (type, model, etc.) if I want to purchase
one to try out?

Is that all a rubber ducky antenna is? 48" of wire wound around a
flexible core of some type?

No, that isn't all that a 'rubber duck' antenna is, wish it were
that simple. Since the question was asked, I have to assume that you
aren't exactly familiar with what a transmitter 'looks' for in an
antenna, or how that 'looked for' thing is arrived at. (And if you
aren't sort of 'involved' in radio stuff, why should you be?) Once
you learn about all that stuff it really isn't all that hard to
figure. But that 'learning' can certainly be a real chore!
- 'Doc

....Rats, that didn't post like I thought it would. (Not enough
coffee.) Disregard the part with the "" thingys. Sorry 'bout that.
- 'Doc

....aw @#$%. That didn't work either. Not my day, it seems. Take it
for what it's worth, disregard the rest, or the whole thing for that
matter. I give up.
- 'Doc

wrote:
...aw @#$%. That didn't work either. Not my day, it seems. Take it
for what it's worth, disregard the rest, or the whole thing for that
matter. I give up.
- 'Doc

Not sure what posts you're referring to, Doc. But
yours look fine to me. No odd arrow charcters.

Regarding this antenna for the low power transmitter,
I expect there will be a bit of grief coupling a
rubber duck, or anything else, in place of what
was the mic cable.

Are they somehow using the shield of the cable
as the antenna? If so it's likely floating and
not directly connected to circuit ground. Plugging
a duck in place of the mic, with typical connectors
and adaptors, would likely not connect the antenna
element to the "antenna terminal" of the device.

I have no doubt that modifying the antenna would
alter the intended tuned antenna circuitry as
well as violate the type acceptance.

Perhaps a bit more about "what is the intended use"
from the OP would help.


Lumpy

You Played on Lawrence Welk?
Yes but no blue notes. Just blue hairs.

www.LumpyGuitar.net

Lumpy wrote:

Regarding this antenna for the low power transmitter,
I expect there will be a bit of grief coupling a rubber duck, or anything else, in place of what
was the mic cable.

Are they somehow using the shield of the cable as the antenna? If so it's likely floating and
not directly connected to circuit ground. Plugging a duck in place of the mic, with typical connectors
and adaptors, would likely not connect the antenna element to the "antenna terminal" of the device.

Thanks, Lumpy. Now we're getting somewhere.

Yes, they are using the mic cable shield for the antenna. Cable is 48"
long with a 1/8" phone plug on the end. The shield side of the mic
connector is floating, that is correct.

I would be connecting the antenna to a new socket of some sort. A BNC
if I can make that fit this rather smallish casing.

Can you give me any starting point for the antenna itself? If I was to
connect it to the existing lead off the coil, the one that now goes to
the mic shield... remove if from there and connect it to the center pin
on the antenna. Would the outer ring of the BNC go to battery negative
ground?

The manufacturer mentioned using a 5K resistor on each side as I recall.
Does thie affect the gain in any way?

scooterspal wrote:

Yes, they are using the mic cable shield for the antenna. Cable is 48"
long with a 1/8" phone plug on the end. The shield side of the mic
connector is floating, that is correct.

I would be connecting the antenna to a new socket of some sort. A BNC
if I can make that fit this rather smallish casing.

Can you give me any starting point for the antenna itself? If I was to
connect it to the existing lead off the coil, the one that now goes to
the mic shield... remove if from there and connect it to the center
pin on the antenna. Would the outer ring of the BNC go to battery negative
ground?

The manufacturer mentioned using a 5K resistor on each side as I
recall. Does thie affect the gain in any way?

I'd only be guessing. Perhaps the 5K resistor(s) are
part of the 'float'.

What kind of signal are you going to transmit
if you aren't using the mic? Just answer generically
if you're an Al Qadea operative or something. ie where
on the gizmo will you connect the audio(?) that you
want to transmit?

If you're injecting the signal somewere else besides
that mic jack, try just temporarily clipping a 48"
piece of wire to the OUTSIDE of the mic jack or whatever
terminal connects to the shield of the mic cable (your coil?).
If that seems to work, then consider your rubber duck or other
options.

The mic jack may be designed to shut power off when
disconnected. You might need to plug a dummy phone
plug into the jack to get it to power on.

Just connecting random wires or antennas to the thing
won't damage anything as long as you only connect to
one point (don't short two terminals together). I don't
think I'd be connecting any antennas to power ground
unless I was really sure of how the thing works.

The outer ring of a rubber duck does essentially
nothing on most rubber ducks.

As others have mentioned, you're probably going to
get LESS of a signal by going to the shortened
duck antenna over the original 48" wire.

Is there a schematic somewhere?


Lumpy

You played on "The Love Boat"?
Yes. White tux, huge sideburns.

www.LumpyGuitar.com

Lumpy wrote:


I'd only be guessing. Perhaps the 5K resistor(s) are
part of the 'float'.

What kind of signal are you going to transmit
if you aren't using the mic?

The original use for the device was to transmit from
someone speaking to someone who was hard of hearing.
Assistive listening is what it's called, I believe.

The same company offers essentially the very same product
WITH the antenna for sending a progam feed from an audio
mixer to the producer or director during a film or video shoot.

That's my need. I'm simply patching the output of my portable
mixer into the existing mic jack on the unit. I'm doing away
with the 48" extended mic cable and the antenna that is part of it.

Holding out 4' of wire will not work for my needs. I need to use
a fixed antenna.

Will any antenna designed for 75-76mhz work?