The AM Forum

* A famous line by TimTron whenever someone tunes up on his frequency... ;D

Well, in the quest to get my SSB rig running again, I blew out my Bird dummyload. Running tone tests. The oil started leaking out and it reads 400 ohms. Toast.  It was a nice load.

So I built up a lightbulb load to get me by using nine 90 watt floodlights.  That's about 800 watts.

I put three in series and these three strings in parallel. The result is the same impedance of one floodlight. I fired it up and ran SWR tests. As expected, as it heats up the impedance drops. After a few seconds, it settles down to a 1.2:1 swr on 75M and 1:1 on 40M and 1.3:1 on 160M.  No input L/C network needed.

The current of 1KW for an inline fuse to protect the bulbs:
I = SqRoot (P/R)  =    SQR(1000/50) = 4.5A =  5 Amp fuse for 1KW pop.

If I make a mistake and blow something out again, these bulbs are easily replaceable. Granted it won't handle 1500 watts or more, but this is OK for now when testing on lower taps.

T

Pictured below, 75 watts from an FT-1000D.

Keep your receiver on when you're using that. You still may be able to make a few contacts.  ;D

You could Shield the Dummy load, small mesh, panels w/holes, etc. many Dummy loads are just mounted in
a sheilded box. I've tried measuring the RF signal inside and outside homemade shielding with a military
grid-dip-meter, Shielding seems to work fine.
A Reciever sitting next to the transmitter will pick-up the VFO, even without transmitting,
 (typical and used  for Spotting the reciever.)
I used a whole shop floor full of large military lamps for the Dummy Load for a 150 Kilo-Watt high freq. switching,
DC power supply (not RF output), worked great,24 hours,days on end, full 150KW output.
My ham dummy load is using 200 watt, continuous duty, non-inductive resistor inside metal sheilding, Dry-load,
No oil needed.
73's  AG5UM

Yep, roger on dummyload radiation with unshielded loads.

I think it is down at least -70 DB on received signals. But I'll get some opinions from the guys tmw on a quiet day and see for sure. It's easy enuff to make a shielded cabinet.  

I learned my lesson years ago. Reminds me of a story from 1988:  I had just finished building a 4-1000A plate modulated rig. I was having trouble getting the audio right and spent the better part of the day testing into a 24 bulb dummyload. It was outside on the ground, unshielded. I had no idea that it could be radiating. There I was, on 3885 all day - Yallo, 1-2-3-4-5, yallo, hello raaaaadio, 1-2-3-4-5... on and on.

The next day I got the rig on the air "for real", so I thought. Mark, WA1FAF asked me what the hell was I doing all day yesterday... I had QRMed a few QSOs and people thought I was crazy... :-)   He said I was at least S9 +10 over all day 100 miles away.  I wanted to crawl under the floorboards..   :-[

T

yo Tom .... you could take those 9 floods and rig an enclosure and a pull out tray and presto !   a fast heating salamander

Pondering how an unshielded dummy load could radiate, I have a few thoughts as to how this may occur.

Even if the load could be made infinitely small, it would still be considered as an isotropic radiator. But, due to its practical size as constructed, the amount of radiation must exceed this level, less the amount of power dissipated in the load. So, the loads physical size radiates as an antenna.

Radiation from the outside of the coax shield will occur as it does with practical antennas which are designed to radiate. Who bothers to install ferrite chokes at the feedpoint of an unshielded dummy load to reduce this radiation? Typically, an antenna presents a low impedance and accepts the bulk of the power sent to it. The outside of the coax presents a much higher impedance and accepts power in inverse proportion to the impedance ratio between itself and the antenna. While radiation from the outside of the coax may go unnoticed when an antenna is used, it becomes an issue when using an unshielded dummy load.

These are the only mechanisms I can identify that relate to the construction of the dummy load. Radiation from the transmitter's (presumably shielded) enclosure, and wiring that penetrates it, is yet another source of radiation.

In sum, these sources of unintended radiation can lead to what is technically known as... embarrassment.  :-[


Don

Easy bake oven which requires long old buzzard transmission to cook pizza pie.

no problem for the Timtron  ;D