On 03/08/14 09:11, Stuart Longland wrote:
In the interest of science, I gave it a shot just then. With a 100nF
capacitor in series to block the DC, I wired it to a DIN5 plug (all my
radios have been set up with adaptors to DIN5 headset jacks) and tried it.

It did work, but without any amplification or impedance matching, the
modulation is well down. I might try winding a small transformer and
see what that does.

Well, I did some math, to transform a 32ohm load to ~600 ohms, I need a
turns ratio of ~4.3:1.

math.sqrt(600.0/32.0)
4.330127018922194

I played with this a little, and so I thought I'd add a couple more,
make it a nice round number. 4.5:1 would give me a 648ohm load on the
HV side.

I had a few L8 toroid ferrite cores laying around, these to be precise:
http://www.jaycar.com.au/productView.asp?ID=LO1230

Not being sure about the number of turns, I made a guess at 45:10, and
started winding.

Just tried it and now the audio is completely dead. Transformer loss is
too high for the feeble signal out of the "microphone".

My guess is that the turns count is waay too small, and that maybe
450:100 might be closer to the mark, but I really don't feel like
winding that many turns on a toroid. 45 felt like a marathon.

So for this to work, I need an in-line amplifier of some sort. My
challenge is to RF-harden it, and have it compatible with both dynamic
microphone inputs as well as electret: as the same headset will probably
be used with both. (If the phone rings and I'm on the bike, I will
sometimes plug the helmet into the phone to answer it. Right now all
headsets are interchangeable with all devices, a feature I'd like to keep.)

I'm thinking possibly a FET, since that's what's embedded in the
electret capsules, however I suspect this will be prone to the same problem.

Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.

Stuart Longland wrote in news:kea1bb-
:

Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.


Did my post earlier not help? If your equipment has a frame ground and a
signal ground, or perhaps an analog plus a digital ground, that might help.
Your CAT5 has no screen so far as I know, it's just 4 twisted pairs. But if
you handle the twisted pair as usual, referenced to signal ground, catch the
RF on the screen and send it to the other ground. if you already tried this,
what happened?

Lostgallifreyan wrote in
:

But if
you handle the twisted pair as usual, referenced to signal ground, catch
the RF on the screen

Meaning a screened mic cable. I said it badly there, thinking of the context
in my earlier post which did mention it directly..

On 03/08/14 19:12, Lostgallifreyan wrote:
Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.

Did my post earlier not help? If your equipment has a frame ground and a
signal ground, or perhaps an analog plus a digital ground, that might help.

I did see your post… basically the frame is purely for the antenna
counterpoise, and nothing else.

Yaesu do connect their negative supply to the antenna ground, which
yields a negative earth, however I do not use this for any kind of DC
path. Not intentionally anyway. (It did happen by accident one day:
+12v came in contact and nearly started a fire. Oopsie!)

Unfortunately, I do not get provided with separate analogue and digital
grounds. I suspect in the set they are one in the same.

Yaesu do supply a separation kit: it basically consists of a mounting
bracket for the head unit, a RJ11-RJ11 round cable for the head unit, a
RJ45-RJ45 flat cable for the microphone and a RJ45-RJ45 adaptor so you
can plug the handmic in.

For the RJ11, I have no idea what the exact pinout is. On that cable
would be serial data (tx/rx), power (probably 5V), speaker output, the
"power/fast tune" button and a signal ground. It'd be nice to know
which one is which but I'm guessing Yaesu probably want to keep that secret.

For the RJ45, it's documented in the handbook. There is +5V, signal
ground, open-collector inputs for PTT, Up, Down and Power/Fast Tune,
then microphone + and -. The microphone input is a nominal 600ohm
impedance, intended for a dynamic microphone.

For convenience, I run 3 lengths of CAT5 with DB25 connectors at each
end. At the ends I then plug in a suitable break-out cable which maps
pins on the RJ11/RJ45 connectors to pins on the DB25.

This is because the cables are frequently connected and disconnected,
particularly on the front basket as I park my bike outside. I found
RJ11 and RJ45 connectors became unreliable when they were used in this
manner. DB25s have been good and reliable however.

I did try wrapping this triple-CAT5 cable in adhesive aluminium tape and
grounding that to the frame, but this did not help. I also have fairly
sizeable ferrite cores clipped on.

I could try running a separate screened cable just for the microphone,
I'd probably have to have a separate connector too. I'll have to
procure parts to do this.

Stuart Longland wrote:
My guess is that the turns count is waay too small, and that maybe
450:100 might be closer to the mark, but I really don't feel like
winding that many turns on a toroid. 45 felt like a marathon.

Don't you have an ancient defective portable AM radio (you know, the
kind that proudly mentioned "6 transistors" on the case)?

The have those small E-I core transformers (soaked in wax) that are
suitable for the job.

Stuart Longland wrote in news:kea1bb-
:

Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.


Did you try ferrite beads? To stop propagation along a screen? No-one seems
to have mentioned them yet so I thought I should...

On 7/27/2014 6:21 AM, 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?

Don't forget that you may need to match the acoustic aspects of the
earpiece to that of a mic meaning you will need at minimum to open up
the opening and preferably add some sort of cone or horn. Poor acoustic
coupling will diminish the signal level just as easily as poor
electrical coupling.

--

Rick