[Yaesu] FT-ONE - low output.

[Dr. Gerald N. Johnson]

On 6/1/2011 11:23 AM, Dr. David Kirkby wrote:
> On 05/29/11 12:49 PM, Rob Atkinson wrote:
>> I don't argue about the Bird accuracy but if you don't like a 43, how
>> will
>> you be content with any hammy hambone watt meter out there marketed to
>> hams?
>
> I think my point is that hams don't really require great accuracy in
> power measurement.
>
> Unfortunately I've never tested any ham meters in a lab, so I don't know
> how good/bad they are.

Then how do you KNOW some Birds won't do 10% accuracy?

Some copies of the 1956 vintage Monimatch are often sold for CB. Near 
all including Bird and Collins wattmeters have an adjustable resistor 
for calibration. Bird pots are hidden in the slugs generally. Many CB 
SWR bridges with watts indications have a serious sensitivity to applied 
frequency.
>
>> If you want to achieve accuracy on a budget, consider getting a
>> thermocouple
>> RF amp meter (they show up at hamfests for a few dollars/pounds/euros but
>> are often hidden under junk because most hams don't give them much value
>> which is good for you) that measures 6 A and a good 50 ohm non-reactive
>> dummy load. Mount the meter in an aluminum box in series with UHF jacks
>> mounted on the sides. Measure your current into 50 ohms and calculate the
>> power. Actually with your legal limit a 2 or 3 A meter is fine.
>
> I don't have much clue about how accurate that would be. I'm not an
> expert in RF power measurement, but when it comes to Bird meters, I
> probably know more than 99.99 % of hams. At least one review on eHam
> describes the meter as "lab grade". The reviewer is seriously mistaken.
>
> I think my main issue with Bird is the claimed accuracy and the cost. If
> the accuracy was as good as Bird state, then I'd have no problem with
> the cost.
>
> But when Bird claim +/- 5% of FSD, and I know some don't even meet 10%
> of FSD, then I do have a problem with the meter.

The slugs are destructible and can be misapplied with the wrong line or 
meter. Especially low power slugs. Coupler based wattmeters are limited 
in their directivity. Using a lower power slug for reflected power tends 
to destroy the low power slug and to contribute to considerable error 
because even with a perfect load the 50 watt slug set for reversed power 
seeing a KW forward power senses that forward power and with directivity 
about 30 dB shows 1 watt reflected for every forward watt. The lower 
power slug has tighter coupling to the line and so when applied for more 
sensitive reflected power can lead to excess dissipation in the coupled 
line termination and detector leading to damage of the slug. That's a 
destructive misapplication and why I no longer consider purchasing low 
power slugs at a hamfest or epay.
>
> I've often wondered how difficult it would be for hams to make a water
> calorimeter. For high power, that's how its measured in a standards lab.
> Of course, this would require that you transmit for a time sufficient to
> reach thermal equilibrium, which would exceed the duty cycle of most ham
> rigs. Making one would be a major project, but an interesting one and
> within the capabilities of a dedicated constructor.

There are two schemes for water calorimetry. One used occasionally for 
primary standards uses controlled water flow through a well insulated 
load and measures the temperature rise of that water flow.

A better ham solution uses the load in a well insulated water volume. 
With that water stirred to keep the temperature uniform, its easy enough 
to compute the rate of temperature rise by the power applied. 3.412 BTU 
per watt. I believe the fundamental is that 1 pound of water rises one 
degree F for each applied BTU, though that may off by a factor of a 
thousand. However its easy enough to measure the rise in water bath 
temperature vs time with applied RF power. Then to let the water cool, 
and apply easily measured DC or power line frequency AC power to the 
same load and adjust for the same rate of water bath temperature rise. 
The insulation can be a $3 foam picnic cooler, the load can be a coil of 
small diameter coax immersed in the water in the cooler. A rapid 
response dial thermometer cost me $7 in the hardware store, should I 
want to do that I keep a couple on hand for checking my vehicle's AC 
performance and when farming for checking soil temperature when planning 
for planting. By calibrating with DC, the thermal mass of the load and 
the water is combined so the experimenter need not measure the volume of 
the water precisely. But the technique gets RF power measurement down to 
the fundamentals of measuring time and temperature. And with the DC 
calibration the precision is limited mostly by the operator care and the 
calibration of the DC instruments which can be traced to primary world 
standards if need be.
>
> I think for me a spectrum analyser is the easiest solution. It may not
> be the best, but is good enough. I could calibrate a ham meter against
> that.
>
Being that the spectrum analyzer is essentially a logarithmic scanning 
receiver with tolerances of a few dB, I'd not consider that precise 
enough for a calibration standard. I much prefer a good (TEK or 
HP/AGILENT only) oscilloscope. I own a Tek 475, the 465 is great for HF, 
like its direct competition the HP1740. I'm less of a fan of digital 
scopes, but the makers don't build analogue scopes anymore.
>
73, Jerry, K0CQ