Post on 02-Oct-2014
description
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PROJECT:
A Super Simple, 50ohm H.F. Salt Water - Dummy Load.
An article which appeared in the Nov.2009 issue of "SHACKNEWS",
the monthly Newsletter of the S.A.,"Highveld Amateur Radio Club" (
the link drawn to my attention by Pieter ZL1PDT), described using a
glass jar containing a salt water (or alternatively washing soda)
solution electrolyte, as a dummy load. I thought to myself, "This is so
too simple, there must be a catch?"
I own a perfectly good commercial grade "Marconi" Dummy
Load/Power meter, but decided to construct and test the device
described in the article. I accessed our Club's MFJ-259 antenna
analyzer, to set up and document the results.
Most Hams will be aware, that a Dummy load cooled with a liquid, can
also be used to accurately calculate the power being sunk into it, by measuring the heat rise in a
given volume, over a given time (1 litre of water, raised 1deg.C per hour = 1.16watts). In this case I
did not have access to a suitable probe and digital meter, nor a suitable glass stem mercury
thermometer and I have not carried out that exercise, but the option is there if desired.
For the container, I selected an empty "Bushalls Coffee" glass jar with
a plastic screw-on lid , identical to that shown in the original article .
This jar, with the lid on, measured 160mm high x 90mm dia. I
removed the inner cardboard liner from the lid, then drilled and
fastened an SO-239 coax socket in the center. Another hole was
drilled in the lid to fit a standard blank rubber grommet into. This
hole is to give access into the jar for adding salt, stirring and later for
measuring the liquid temperature if desired. An added benefit would
be to allow steam to pop the grommet and escape should one do the
unthinkable and boil the water!
One of the SO-239 socket fastening screws was left long and a length of stiff 16# copper tinned
wire was attached with a second washer and nut. Another short
length of the same wire was soldered to the center of the fitting.
Both wires were then bent outwards away from each other and
down, sprung apart to be as close as possible to the glass on each
side. The wires were then cut to length, approx. 5mm from the
jar's bottom . A liberal coating of silicon sealant was then applied
to the underside of the SO-239 socket and around the fastening
screws, washers and nuts, to minimize corrosion from the
saltwater solution. Although I did not do so, it would be
worthwhile to make and fit a rubber gasket between the lid and
the top of the glass jar.
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The next step was to fill the jar with clean tap water, to the level coinciding with the bottom of the
lid's vertical side. Next I measured the SWR. I used the MJF-259 Antenna Analyzer (but a signal
source of a few watts through a good SWR meter will do the same job).
The SWR was very high (between 15:1 and 28:1 over all the HF bands). A tiny amount of salt was
introduced through the grommet hole in the lid and into the water, the water was stirred with a
spatula and another SWR reading taken. Repeating the process, the SWR gradually came down as
tiny amounts of salt were added, stirred and tested. Until I achieved a feel for what was happening, I
found a couple of times that I had over salted and then had to remove some of the solution, adding
in fresh water to top up and then carefully bringing the salinity up again. Eventually the SWR came
down to 1.1:1 SWR from 160m down to 20m, 1.0:1 SWR from 17m down to 12m, then back up to
1.1:1 SWR at 10m , the total salt used was approx 1/8th of a teaspoon. If, over a period of time the
water level dropped, one would simply add more clean tap water, no more salt or testing should be
required.
The grommet plug was then fitted and it was time to give it the smoke/steam test!
I duly connected my FT-897D in cw mode, with a coax jumper cable directly to the top of the lid and
applied 10watts at 1.850mHz, then checked each band successively up to 30mHz, all was sweet, with
a near perfect SWR reading registering on the transceiver's meter. I then tried the same procedure
with 50watts of power, then with 100watts for approx. 5 seconds on each band . All was good, so a
prolonged test was done with 100watts at 17m (18.110mHz) for 5 minutes, after which the glass jar
was only slightly warm but not hot.
With a little outside assistance, I was able to check for RF
radiation whilst using the device as a dummy load. This
involved listening on the appropriate frequency to a
digital "Sony ICF-2001" portable communication receiver
with integral whip antenna, outside and about 20m from
the shack, disappointingly a weak cw tone signal was
heard. A further test was done by holding a field-strength
meter with whip ant. about 1 meter from the jar whilst
transmitting, the meter indicated a low level of radiation.
To remove all radiation, I wrapped the jar completely with
aluminium foil, making sure it also made contact with the
coax connector on the jar top. Another test transmission
was made and even with the Field Strength Meter antenna 10mm from the wrapped jar, no signal
registered.
Unfortunately, wrapping the metal foil around the jar also raised the SWR to an unacceptable level,
due I believe to the capacitor effect with the foil so close to the electrodes, only separated by the
jar's glass walls.
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Having decided that a dummy load which radiated
even a very weak signal was not acceptable, I
purchased a new 2L unused paint tin to act as a
screened enclosure, with sufficient internal
clearance to have a reasonable air gap between the
jar and the tins inside surfaces. The coffee jar I had
used to house the "dummy load" was too tall and a
shorter coffee jar was modified to become the new
"Dummy Load." This time, the build up of salinity to
obtain a low SWR, was done with the new jar
fastened to the underside of the paint tins lid with
the lid pressed lightly home into the paint tin. A suitable hole was installed to add salt and stir, later
covered with a blank grommet.
The SWR results obtained with the analyzer were even better then from the first version, 160m,
80m and 40m showed a 1.1 to 1 SWR. All the remaining bands, 30m, 20m, 17m, 15m, 12m and 10m
showed a 1.0 to 1 SWR. Power tests followed and thorough testing showed the complete absence
of any leakage radiation! As the first version had worked so well in the key down tests, this time I
applied 100w, cw mode, at 3.6mHz for 2.5 minutes, then immediately changed frequency to
28.4mHz and keyed down a further 2.5 minutes. Removing the lid and holding the jar showed that
even though a smaller volume of saline solution was involved, after the 5min. testing period, the jar
was quite warm but still far from hot.
To conclude, the device works surprisingly well as a "50ohm Dummy Load", the junkbox provided
the materials except for the new 2L paint pot enclosure which cost NZ$6-98, plus the "pinch of salt"
purloined from the XYL's kitchen.
Ian, ZL1TXZ.