[TenTec] Model 1225 SWR and Wattmeter Kit

[Bob McGraw - K4TAX]

Carl I totally agree.

Most watt meters are accurate or most accurate at only their calibrated 
impedance. Which in most cases we presume to be 50 ohms.  Thus if one is 
using it with an antenna of unknown impedance, the power value display is 
most likely unknown as  well.  However, using one of these for forward and 
reflected values is usually satisfactory as the SWR value is a ratio.

For fact, my Celwave dummy load is 50.5 ohms while my MFJ gallon of oil 
dummy load is 54 ohms cold and about 58 ohms hot.  The dry MFJ dummy load 
impedance also varies with temperature of the resistor.  With these loads, a 
given watt meter will show different values of indicated power.

Just do the math, being square root of (P x R) to solve for E and one will 
see that the voltage changes with a change in R.  For this reason I prefer 
to measure the voltage across the load and do the math being E squared / R 
for watts.  To measure the voltage I have a HP RF voltmeter which has a 1% 
FS accuracy.  It and the Celwave load gets really close to the real or 
absolute power value.

Calculating power meters such as the LP-100A become a bit more costly.

73
Bob, K4TAX




----- Original Message ----- 
From: "Carl Moreschi" <n4py3 at earthlink.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Monday, July 14, 2014 8:01 PM
Subject: Re: [TenTec] Model 1225 SWR and Wattmeter Kit


>I won't to point out that not all watt meters are created equal.  Most 
>inexpensive watt meters only monitor RF voltage.  They are calibrated 
>assuming the load is 50 ohms.  This is only the case for a perfect 1:1 SWR. 
>More expensive meters like the LP-100A have both current and voltage 
>sensors and read correctly for all loads, not just 50 ohms.
>
> Carl Moreschi N4PY
> 58 Hogwood Rd
> Louisburg, NC 27549
> www.n4py.com
>
> On 7/14/2014 4:17 PM, Jim Brown wrote:
>> On 7/13/2014 7:39 PM, Glenn AE0Q wrote:
>>> I know this is the TenTec list, but since RF Concepts owns both
>>> companies...
>>
>> That only matters if you're a cheerleader. If you're an objective
>> observer, and want the best bang for the buck on a really good
>> wattmeter, look at N8LP's LP100A. Elecraft uses them on the production
>> line to test their radios. I've got an order in for his new one, the
>> LP-500, which can display the output of four SWR couplers. The LP100A
>> has been around for years, and is a mature product. The LP500 has not
>> yet shipped.
>>
>> http://www.telepostinc.com/n8lp.html
>>
>> Note that there's a link at the bottom of his home page to N4PY Software.
>>
>> 73, Jim K9YC
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