[TenTec] Model 1225 SWR and Wattmeter Kit

[Bob McGraw - K4TAX]

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 at earthlink.net>
To: "Discussion of Ten-Tec Equipment" <tentec at 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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