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Idea for an AM field stength meter

R

radio8z

Hello all,

I have been building and testing my AM P15 antenna. I have an idea about a field strength meter.

I have a GE radio (model 7-2664b) which uses a 28 pin single IC. The mfg. has sandpapered the ID off the chip, but I found that the Samsung KA22426 fits the wiring and pin connections. This is probably the chip they used. Link: http://katalog.elektroda.net/download.php?id=50098

This chip has a tuning meter output which in my radio is not connected. I am going to connect a meter to this pin and see if it does give a signal strength reading.

I know from past experiments that this may not work because the AGC may saturate which limits the meter resolution. I have a Motorola "test set" so I can calibrate the meter vs. uV of RF. If I have time I will do this.

This is just an idea right now and I will report what I find in the next few days.

It seems that any cheap AM radio of recent manufacture could be adapted, with a VOM, to serve as a field strenght meter. Not as easy as in the old days when you just hooked the meter across the volume pot for AGC, but still may work.

Neil
 
> Hello all,
>
> I have been building and testing my AM P15 antenna. I have
> an idea about a field strength meter.
>
> I have a GE radio (model 7-2664b) which uses a 28 pin single
> IC. The mfg. has sandpapered the ID off the chip, but I
> found that the Samsung KA22426 fits the wiring and pin
> connections. This is probably the chip they used. Link:
> http://katalog.elektroda.net/download.php?id=50098
>
> This chip has a tuning meter output which in my radio is not
> connected. I am going to connect a meter to this pin and
> see if it does give a signal strength reading.
>
> I know from past experiments that this may not work because
> the AGC may saturate which limits the meter resolution. I
> have a Motorola "test set" so I can calibrate the meter vs.
> uV of RF. If I have time I will do this.
>
> This is just an idea right now and I will report what I find
> in the next few days.
>
> It seems that any cheap AM radio of recent manufacture could
> be adapted, with a VOM, to serve as a field strenght meter.
> Not as easy as in the old days when you just hooked the
> meter across the volume pot for AGC, but still may work.
>
> Neil
>

Excellent idea Neil.

I agree that the strong signal close to the antenna may likely overload the receiver and flatten the peak meter reading. This can possibly be minimized by orienting the radio so its directional antenna receives the minimum level signal.

Virtually everyone owns a potential field strength meter. All AM radios have the AVC function ("automatic volume control"). They all have a point in the circuit where you can measure the AVC voltage. The only trouble is, you may have a difficult time finding that point without a circuit diagram. A cheap multimeter from RadioShack is all you need to measure it.

There are two general types of field strength meters: calibrated and relative. A calibrated FS meter would be great to have to test your signal against FCC limits, but they are super expensive. Relative FS meters just give a meaningless reading on a meter. But the "meaningless" reading is actually quite useful for tuning your antenna. The important thing is to watch the meter as you tune. The voltage will peak at the optimum tuning point and drop on either side.

A field strength meter is absolutely the best way to confirm your antenna is tuned to maximum. Back in the '70s when the CB radio fad was at its peak you could buy inexpensive relative FS meters everywhere. Now, cheap ones are rare.

MFJ is one of the few sources now for inexpensive relative FS meters. Check:
http://www.mfjenterprises.com/products.php?prodid=MFJ-801
http://www.mfjenterprises.com/products.php?prodid=MFJ-802
<P ID="signature">______________
Phil B
</P><P ID="edit"><FONT class="small">Edited by Phil B on 10/08/05 05:18 AM.</FONT></P>
 
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