On Sun, 27 Jul 2014, Stuart Longland wrote:

Hi,

Silly question? I've been experimenting with a radio station on the
bicycle for HF operation and one problem I've been facing is that of RF
feedback getting into the microphone.

I've tried a few things to try and eliminate the RF. The set is a Yaesu
FT-857D, and I use runs of CAT5 cabling to hook the head unit and my
headset up to the radio which is mounted in a motorcycle topbox behind
me. I've ensured that the microphone + and - signals are on their own
dedicated pair in the CAT5, as are the +5V and GND signals.

The microphone biasing is done near the head unit of the radio, so maybe
1.5m away from the radio, and a short lead then plugs into the
(helmet-embedded) headset. My biasing circuit looks like this: (please
excuse the ASCII art)
.-----. .---------------~~----.
+5v ---o------------o----| 1k |---o ..100nF |
| | '-----' '---||--o---o Mic + +| .
----- ----- '' --- ( )| Electret
.-'-. 100uF ----- 100nF ,. --- 100nF -| ' Insert
| | .-----. .---||--o---o Mic - | (headset)
0v----o------------o----| 1k |---o ''100nF |
'-----' '---------------~~----'

The capacitors are all (with the exception of the 100uF electrolytic)
small monolithic capacitors:
http://www.jaycar.com.au/productView.asp?ID=RC5490

My intent is that the capacitors across the 5V rail should suppress the
RF on the 5V rail, and the capacitor across the output of the bias
circuit should suppress any common-mode RF on the balanced feed to the
radio. I think there's one across the microphone in the headset too.

Originally I contemplated a phantom-power arrangement, but I'd need to
sit down and design the bias circuit as most of my stuff is in the 3-5V
range, not the 12-48V range that phantom-power normally assumes.

I note I do not get the RF feedback if I use the stock Yaesu handmic
that came with the set, which is a 600ohm dynamic microphone.

Currently I use an electret insert in the headset, one out of the junk
box often salvaged from old mobile phone "headsets" (those annoying
earphone things with an inline microphone).

The lack of problems with the Yaesu handmic got me thinking: I have
amassed a large quantity of earphone speakers from those mobile phone
headsets (I hate the bloody things: can't stand putting anything *in* my
ears).

These are speakers with dimensions not too dissimilar to small dynamic
elements with an impedance of 32ohms. Question is, how well would these
work as microphones? Has anyone tried this and can share any insights?

IN the old days, endless cheap 100mW walkie talkies would use the speaker
for the microphone on transmit. For that matter, endless intercom systems
used the same speaker as a speaker and as a microphone.

Endless articles in the magazines showed taking a speaker of some sort and
using it as a microphone.

Of course, those were speakers, most seemed at least 2".

An earphone may not offer the same level of sound collection that a
speaker with a larger cone allows, you may have to play with things. I
remember taking cheap dynamic earphones and taking the bit that went in
your ear off, and using that as a contact microphone for various things.

The other issue is the output may not be so great, mostly because the
speaker is arranged for sound reproduction, not sound pickup. It may need
a preamp, depending on the actual output voltage. It may also need
impedance matching, which would also help boost the output, use an output
transformer in reverse so the speaker winding is fed from the speaker, and
the now secondary winding will have more voltage to feed the higher input
impedance of the rig.

Michael