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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.