[TenTec] Model 1225 SWR and Wattmeter Kit

[Steve Hunt]

Some care has to be taken when interpreting the LP-100A's power readings.

On my version it indicates Pfwd when the "F+R" mode is selected, and 
Pfwd-Pref when the "Net" mode is selected. I got caught out on several 
occasions assuming the "F+R" mode meant Pfwd+Pref - it doesn't!

Note also that the derivation of Pfwd and Pref from the Voltage and 
Current sensors assumes a 50 Ohm Zo; I don't know any way to alter the 
characteristic impedance that the LP-100A assumes, unless it's buried 
deep in the menus and I haven't found it?

Steve G3TXQ




On 22/07/2014 13:03, Carl Moreschi wrote:
> The LP-100A has two connections to the sensor.  One is for current and 
> one is for voltage.  Power is simply the product of voltage times 
> current.  So for any load impedance, power will always be calculated 
> correctly.
>
> Most less expensive meters only measure voltage.  They calculate power 
> as Voltage squared divided by 50.  They always assume the load is a 
> pure 50 ohms.  But with a 1.5 swr, you could have a 75 ohm load or a 
> 33 ohm load or a complex load with both real and imaginary 
> components.  If you have a 33 ohm load, your power will read high.  If 
> you have a 75 ohm load, your power will read low. The point is unless 
> the swr is exactly 50 ohms, your power measurement will be wrong by a 
> lot.
>
>
>
> Carl Moreschi N4PY
> 58 Hogwood Rd
> Louisburg, NC 27549
> www.n4py.com
>