Antenna monitor for low power TX
 

Hi,

I am looking for some ideas for a low cost circuit to detect if an antenna
has been broken or removed
from a 10mW transmitter.

Initial thoughts are to measure SWR but I need a small, low cost circuit I
can place between PCB Antenna connector and the antenna.

Thanks in advance for any assistance

Kind Regards

David Huisman

I am looking for some ideas for a low cost circuit to detect if an antenna
has been broken or removed
from a 10mW transmitter.

Hi David, The best, low cost idea that comes to mind is look and see if the
antenna is gone.
I don't think that was what you were looking for. What you need is something
like the circuits in solid state HF rigs that reduce power when they sense a
mismatch. At 10mW you will need additional circuitry to boost the signal up to
a level that the sense circuit can use.
All the circuits I know about will cost quite a bit more than your 10mW
transmitter. Out of curiosity, what are you trying to do?
73 Gary N4AST

David,

If your antenna has DC continuity, put DC on the feedline through an RF
choke and resistor. Do it like this: DC voltage to resistor to RF choke to
center conductor of coax. Red LED from junction of resistor and RF choke to
shield of coax. The red LED will come on when the path is broken. If the
antenna normally does not provide a path, connect anothe RF choke at the
antenna, in such a way that whoever steals the antenna also takes the RF
choke. You want the RF chokes to be PARALLEL resonant somewhere just above
the operating frequency.

Tam/WB2TT
"David Huisman" wrote in message
...
Hi,

I am looking for some ideas for a low cost circuit to detect if an antenna
has been broken or removed
from a 10mW transmitter.

Initial thoughts are to measure SWR but I need a small, low cost circuit I
can place between PCB Antenna connector and the antenna.

Thanks in advance for any assistance

Kind Regards

David Huisman

"David Huisman" wrote in message
...
Hi,

I am looking for some ideas for a low cost circuit to detect if an antenna
has been broken or removed
from a 10mW transmitter.

Initial thoughts are to measure SWR but I need a small, low cost circuit I
can place between PCB Antenna connector and the antenna.

Thanks in advance for any assistance

Kind Regards

David Huisman


I'm the lazy sort. I'd get a 1N34 diode, a capacitor, a resistor, and either
a surplus microammeter or one of those cheap little digital panel meters.
Build an RF voltmeter. 10 mW into a 50 Ohm load should cause about 1 V of RF
when peak rectified. That's enough to read on a meter through the diode. It
doesn't matter what the meter reads--just jot down the value when you know
the antenna system is right and then watch for changes.

You must be a QRPer. Someday I hope to get my homebrew Micromountaineer (.5
W) on the air.

Have fun.

John

I am unable to use DC continuity. Even if I had a termination at the antenna
base , this would only indicate broken cable, but not a cut antenna. The DC
path would need to be to the tip of the antenna.

I have some 10-bit analog inputs available for monitoring and can resolve
down to a couple of mV.

Any other ideas ?

Thanks

--

Kind Regards

David Huisman "Tarmo Tammaru" wrote in message
...
David,

If your antenna has DC continuity, put DC on the feedline through an RF
choke and resistor. Do it like this: DC voltage to resistor to RF choke to
center conductor of coax. Red LED from junction of resistor and RF choke
to
shield of coax. The red LED will come on when the path is broken. If the
antenna normally does not provide a path, connect anothe RF choke at the
antenna, in such a way that whoever steals the antenna also takes the RF
choke. You want the RF chokes to be PARALLEL resonant somewhere just above
the operating frequency.

Tam/WB2TT
"David Huisman" wrote in message
...
Hi,

I am looking for some ideas for a low cost circuit to detect if an
antenna
has been broken or removed
from a 10mW transmitter.

Initial thoughts are to measure SWR but I need a small, low cost circuit
I
can place between PCB Antenna connector and the antenna.

Thanks in advance for any assistance

Kind Regards

David Huisman

I am looking for some ideas for a low cost circuit to detect if an antenna
has been broken or removed
from a 10mW transmitter.

At that power level why worry about a broken antenna ? The components
should be able to handle the loss . It would probably take more electronics
than it was worth. One idea is to monitor the current in the final stage
and see if it changes with the loss of the antenna.

Just monitor the voltage across the feedline to the antenna,


Use a diode + capacitor + 100K-ohm resistor + microammeter.


It doesn't matter what the actual voltage is. If it changes from normal
then something is wrong.


Or just keep an eye on the DC milliammeter in the final amplifier circuit.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

David,

It would be simpler to swith to an antenna that provides a DC path, than to
build an RF driven detector.

Tam/WB2TT
"David Huisman" wrote in message
...
I am unable to use DC continuity. Even if I had a termination at the
antenna
base , this would only indicate broken cable, but not a cut antenna. The
DC
path would need to be to the tip of the antenna.

I have some 10-bit analog inputs available for monitoring and can resolve
down to a couple of mV.

Any other ideas ?

Thanks

--

Kind Regards

David Huisman "Tarmo Tammaru" wrote in message
...
David,

If your antenna has DC continuity, put DC on the feedline through an RF
choke and resistor. Do it like this: DC voltage to resistor to RF choke
to
center conductor of coax. Red LED from junction of resistor and RF choke
to
shield of coax. The red LED will come on when the path is broken. If the
antenna normally does not provide a path, connect anothe RF choke at the
antenna, in such a way that whoever steals the antenna also takes the RF
choke. You want the RF chokes to be PARALLEL resonant somewhere just
above
the operating frequency.

Tam/WB2TT
"David Huisman" wrote in message
...
Hi,

I am looking for some ideas for a low cost circuit to detect if an
antenna
has been broken or removed
from a 10mW transmitter.

Initial thoughts are to measure SWR but I need a small, low cost
circuit
I
can place between PCB Antenna connector and the antenna.

Thanks in advance for any assistance

Kind Regards

David Huisman

"David Huisman" wrote in message ...
I am unable to use DC continuity. Even if I had a termination at the antenna
base , this would only indicate broken cable, but not a cut antenna. The DC
path would need to be to the tip of the antenna.

I have some 10-bit analog inputs available for monitoring and can resolve
down to a couple of mV.

Any other ideas ?

Thanks

You didn't mention the frequency, but monitoring the reflected power
should be no big deal. A 20dB directional coupler would waste only
100uW with a proper load connected, and even without resonant
enhancement of the reflected-port voltage give you enough to detect a
disconnected antenna. A coupler could be transformer based (a couple
small toroids) at lower frequencies or coupled lines (for example
microstrip, perhaps built into your PC board) at higher frequencies.

Cheers,
Tom

How about a receiver with variable attenuation? so you have a good signal
when all is well, then when you pull the antenna outa the tx'er, the signal
disappears from your receiver.

Gary



"David Huisman" wrote in message
...
Hi,

I am looking for some ideas for a low cost circuit to detect if an antenna
has been broken or removed
from a 10mW transmitter.

Initial thoughts are to measure SWR but I need a small, low cost circuit I
can place between PCB Antenna connector and the antenna.

Thanks in advance for any assistance

Kind Regards

David Huisman