Carl is very correct on these points.
With typical meters that are only voltage sensing, from my experience, power
is best measured into a dummy load of known impedance. As to the station
operation, I am more interested in the reflected amount and in both cases,
FWD and REF become only reference values and not absolute values.
73
Bob, K4TAX
----- Original Message -----
From: "Carl Moreschi" <n4py3@earthlink.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, July 22, 2014 7:03 AM
Subject: Re: [TenTec] Model 1225 SWR and Wattmeter Kit
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
On 7/22/2014 4:37 AM, K8JHR wrote:
Gee... Sorta condescending and demeaning. And not particularly helpful.
I often forget ham radio is only for experts with careers in electrical
engineering - the rest of us hobbyists are merely tolerated, perhaps
only because we buy stuff and keep prices down.
OK, I read the N8LP web page and, I am sorry because I do not see where
it addresses Carl's point about some meters being less accurate if the
load varies from 50 ohms, or how the LP-100 is immune from that problem.
Perhaps someone can explain this on the E-brand reflector... As far as I
can see, both units might be affected similarly as they will be used
exactly in the same way. I suppose I missed it, but I tried.
Sidebar: The web page says the LP-100 is the only meter with
simultaneous bar graphs for both power and SWR... but that is not true,
the W2 has this feature, also. Go figure... ;-)
So, OK, I am getting familiar with the old fifth grade brush off answer,
"...someday you will understand." I appreciate Carl's reply, but I still
don't know how the one meter is more immune than the other. So far, Bob
is the only one to offer a solution within the specifications and budget
stated.
So, never mind, I will drive on, reading my ARRL Handbook, slowly, page
by page, and wait for the great epiphany everyone else must have already
experienced. But I won't buy something just because somebody says I
should.
And while I am at it, this whole "contest" thing seems a bit overblown.
How many contests have been lost because the the operator could not
determine amplifier power with sufficient accuracy? Would a fraction of
a dB more oomph really make all the difference? All this talk about
contesting... I would love to know everyone's actual contest record! I
wonder how many are really in the race. Heck, a lot of TT guys disparage
ICOM radios, but K3LR has a shack full of them. So who can you trust?
Thanks for the courtesy of a reply, and I am sorry I don't get it.
Happy days.
------------------- K8JHR ----------------------
On 7/22/2014 12:45 AM, Jim Brown wrote:
On 7/21/2014 8:27 PM, K8JHR wrote:
Would I really benefit from spending more on the LP-100? That is the
real question. Would it really be worth twice the cost? What would
I do with the more accurate information it provides? Do I really need
more than a close approximation? If so, what would it do for me?
You could study N8LP's web page. Click on the LP100A link. It's worth it
to me if it lets me run closer to legal limit in a contest, or when
calling DX. 1.5:1 SWR causes significant error in the power measurement.
You could learn more about how transmission lines work.
73, Jim K9YC
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