Hi all,

I'm curious about dummy loads. I read that ideally dummy loads convert
RF energy into heat, but I'm sure in the real world, there will still
be a small amount RF energy transmitted.

My friend tried using a dummy load on a 300W transmitter. He was only
running it for a few mins at the VHF range, but he was picking up a
signal 100 ft away. Is that typical?

What's your experience on dummy loads? Were you able to pick up a
signal, as well, with this type of distance?

I don't know much about dummy loads, but I think that 100ft is just a
tad bit too far, no?

CD wrote:
Hi all,

I'm curious about dummy loads. I read that ideally dummy loads convert
RF energy into heat, but I'm sure in the real world, there will still
be a small amount RF energy transmitted.

My friend tried using a dummy load on a 300W transmitter. He was only
running it for a few mins at the VHF range, but he was picking up a
signal 100 ft away. Is that typical?

What's your experience on dummy loads? Were you able to pick up a
signal, as well, with this type of distance?

I don't know much about dummy loads, but I think that 100ft is just a
tad bit too far, no?

Even with a perfect dummy load you'd still get some leakage from your
transmitter and cabling -- I generally expect that if I run into a dummy
load I'm going to peg the S meter of any receivers that are on-frequency
and in the room.

So picking up the signal 100ft away doesn't surprise me.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Doug McLaren wrote:

In article .com,
CD wrote:

| My friend tried using a dummy load on a 300W transmitter. He was only
| running it for a few mins at the VHF range, but he was picking up a
| signal 100 ft away. Is that typical?

Probably

| I don't know much about dummy loads, but I think that 100ft is just a
| tad bit too far, no?

There is the guy who got a Worked all Continents using only a light
bulb as his antenna. (And basically it's just a dummy load, though
the resistance varies and so it doesn't make a very good dummy load,
though it shouldn't make a better antenna than a good dummy load. Was
that sentence long enough?)

The leakage of your rig and the feed lines is signifigant. And of
course, the dummy load itself will radiate a little bit too. You may
have a hard time getting a WAC, but certainly the signal is there and
can be picked up by those close. It's also why it's a good idea to ID
yourself and such even when transmitting into a dummy load.


If I might insert my comments. I whole heartedly agree with the other
responses. A dummy load will radiate and using light bulbs as a dummy
load will work.

However, they will radiate emf as well as light. One of my mother's
cousins, Arnold W2OVP SK, received his first ham license on something
like 8DEC1941, if you are a student of history you know what happened
next. Radio Silence until after August 1945. Anyway one night he was
practicing his CW into a bank of light bulbs at his home in Buffalo, NY.
He received a phone call from one of his friends on the other side of
town that his code sounded good. Suffice it to say that he shutdown and
did not do that again. So yes a dummy load will radiate, just not very
efficiently.

Dave WD9BDZ

On Wed, 07 Sep 2005 07:12:12 -0700, Tim Wescott wrote:
CD wrote:
Hi all,

I'm curious about dummy loads. I read that ideally dummy loads convert
RF energy into heat, but I'm sure in the real world, there will still
be a small amount RF energy transmitted.

My friend tried using a dummy load on a 300W transmitter. He was only
running it for a few mins at the VHF range, but he was picking up a
signal 100 ft away. Is that typical?

What's your experience on dummy loads? Were you able to pick up a
signal, as well, with this type of distance?

I don't know much about dummy loads, but I think that 100ft is just a
tad bit too far, no?

Even with a perfect dummy load you'd still get some leakage from your
transmitter and cabling -- I generally expect that if I run into a dummy
load I'm going to peg the S meter of any receivers that are on-frequency
and in the room.

So picking up the signal 100ft away doesn't surprise me.

A friend of mine and I used to do 'code practice' on 40M with our rigs
cranked down to approx 20 watts each -- using Heathkit Cantennas at both
ends. S-9 sigs both ways. We were 5 blocks apart.

As well, once a week we used to (attempt to) check into a distant (180
miles) 2M SSB. While waiting for net control (and the rest of the net)
to point in our compass direction (220 degrees - approx 35 past the hour),
we'd chat back and forth - again switched into the Cantennas. And, again,
S-9+ sigs both ways.

It's more a testament to the quality of good receivers now-a-days -- than
the leakiness of a dummy load. Look over what the QRP and QRPp folks are
doing - with their "Millions of Miles Per Watt" records, and you'll
realize (if you haven't already) than you can happily 'do' amateur radio
with Not Much Watts.

73
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK

"CD" wrote
My friend tried using a dummy load on a 300W transmitter. He was only
running it for a few mins at the VHF range, but he was picking up a
signal 100 ft away. Is that typical?
______________

If that 300 W was all radiated from a matched 1/2-wave dipole, it would
generate a free-space, maximum field of about 4 volts per meter at a
distance of 100 feet.

If the sum of cabinet, cable and dummy load radiation was 80 dB below that,
the maximum free-space field at 100 feet would be about 400 microvolts per
meter -- a very healthy signal.

RF

My dummy load is 100% enclosed in oil AND a 1 gallon can. This is pretty
tight. I estimate leakage at around -120 to - 130 dB.

I run about 10 to 11 feet of coax from my transmitter to the low pass
filter to the tuner to the antenna switch to the dummy load. The coax is
good quality 9913. BUT, it uses a woven copper braid as the overbraid.
The braid has a leakage impedance of typically 0.020 + jw10^-8 ohms. The
inductive term means that the little holes in the braid 'LEAK' rf. At
VHF the coax isolation could be -60 to -80 dB. So, a 100 watt signal
could leak between 10 and 100 microwatts. If you have a cell phone that
power level is close to your cell phone's power output. Your receiver
will detect a signal 1 one millioneth as strong.


CD wrote:
Hi all,

I'm curious about dummy loads. I read that ideally dummy loads convert
RF energy into heat, but I'm sure in the real world, there will still
be a small amount RF energy transmitted.

My friend tried using a dummy load on a 300W transmitter. He was only
running it for a few mins at the VHF range, but he was picking up a
signal 100 ft away. Is that typical?

What's your experience on dummy loads? Were you able to pick up a
signal, as well, with this type of distance?

I don't know much about dummy loads, but I think that 100ft is just a
tad bit too far, no?

"Ham op" wrote in message
...
My dummy load is 100% enclosed in oil AND a 1 gallon can. This is pretty
tight. I estimate leakage at around -120 to - 130 dB.

I run about 10 to 11 feet of coax from my transmitter to the low pass
filter to the tuner to the antenna switch to the dummy load. The coax is
good quality 9913. BUT, it uses a woven copper braid as the overbraid.
The braid has a leakage impedance of typically 0.020 + jw10^-8 ohms. The
inductive term means that the little holes in the braid 'LEAK' rf. At
VHF the coax isolation could be -60 to -80 dB.

Ahhh! You stole' my thunder. Coax typically only has 60-80 dB isolation at
best and that is if it is made well and connectors are on well. Before it
even gets to the coax connector of the radio, there'll be some radiation.
73, Steve, K9DCI

The RF power at 2W is about +33 dBm. The cable's connector ends are the most
likely place for RF leakage to occur, as it is with the termination's
connector. Suppose the leakage is 80dB. That would make the radiated power:

P = +33dBm - 80dB = -47dBm
(This is right at the connector site.)

The field intensity goes as the reciprocal of the radius squared. Since -47
dBm is about 20 nW and power in Watts implies MKS units,

P (at 100 feet) = 20 nW divided by (30)squared,

since 100 feet equals approximately 30 meters.

This is about 20 pW or -77dBm.

A good receiver can detect down to -120 dBm (I think).

The field strength of -77 dBm is well within the sensitivity of a good
receiver of -120 dBm.

The short answer is that to detect signals 100 feet away is a likely event
with a typical termination load.

"CD" wrote in message
oups.com...
Hi all,

I'm curious about dummy loads. I read that ideally dummy loads convert
RF energy into heat, but I'm sure in the real world, there will still
be a small amount RF energy transmitted.

My friend tried using a dummy load on a 300W transmitter. He was only
running it for a few mins at the VHF range, but he was picking up a
signal 100 ft away. Is that typical?

What's your experience on dummy loads? Were you able to pick up a
signal, as well, with this type of distance?

I don't know much about dummy loads, but I think that 100ft is just a
tad bit too far, no?