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Author Topic: dummy loads  (Read 3565 times)
N4JTE
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Posts: 1154




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« on: August 15, 2013, 06:39:37 PM »

Okay i will throw this on this forum, someone tell me what a dummy load is supposed to do; check a transmitter output into a 50 ohm load, okay but that seems silly, pretty obvious, should work.
I only use a dummy load to cut phase lines but what possible use beyond that can a dummy load be used for in this day of 50 ohm amps, tranceivers etc.
Tuning up with a dumb load has no meaning, the antenna is where the energy needs to be monitored and very easy to accomplish with 5 watts for 2 seconds on that thing in your backyard so what does a dummy load do these days?
Bob
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KD0REQ
Member

Posts: 858




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« Reply #1 on: August 15, 2013, 06:52:05 PM »

obviously, crank out killer watts without putting them on the air.  this is more useful than just boiling the oil if you are doing any repair work on the powered end.  check spectrum, check to see if your tuner is shot, load up those new tubes and see if anything goes bang, see if you actually have power without jamming the band.  if you can load the dummy, but not the antenna, that eliminates one possible failure point.

it's test equipment.  without test equipment, shoot, I hate to say "appliance operator," but...
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K8AXW
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Posts: 3677




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« Reply #2 on: August 15, 2013, 08:10:34 PM »

JTE:  Congratulations!  You're the first "ham" that I have ever heard ask that question in 57 years!
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VU2NAN
Member

Posts: 239




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« Reply #3 on: August 15, 2013, 08:41:26 PM »

Hi OM Bob,

You're right.

A dummy load is only for the 'repair man'.

As an operator, I have never used a dummy load and don't own one either.

But I thought I was stumped when I had to test a friend's SB-200, after setting it right.

For, my antennas were fed with only RG-58/U cable.

Here's how I solved that problem.

http://nandustips.blogspot.in/2011/02/handy-1-kw-hf-dummy-load.html

73,

Nandu.
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N2MG
Administrator

Posts: 122



« Reply #4 on: August 16, 2013, 05:16:15 AM »

Dummy loads are also employed in some antenna designs (see 4-square antennas), but those are rare beasts. 

I'm with you guys on this one!  For the most part a dummy load is just an unnecessary tripping hazard.

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W3DBB
Member

Posts: 72




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« Reply #5 on: August 16, 2013, 06:50:47 AM »

Nandu's hot plate has a resistance of 50 Ohms. This implies the DC resistance is 50 Ohms.

What is the impedance of the heating element at the frequencies the SB-200 is being tuned-up?

Specifically, how many Ohms impedance (Z) and at what phase angle? The reason I ask is because power transfer is also a concern. How inductive is the heating element?

How far does the RF coupled to the hot plate propagate? How well shielded is the hot plate at radio frequencies? What is the resonant frequency of the hot plate?

I had always thought using a well-shielded, reasonably noninductive (at the test frequency) dummy load was the best way to tune up a tube final PA without radiating much of a signal.

In the case of a solid-state transceiver either by itself or driving a solid-state amplifier, dissipating a test signal into a dummy load with an inline wattmeter to verify proper output in the event of poor system (including the antenna) performance is a helpful tool.
 
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KE4JOY
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Posts: 1335




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« Reply #6 on: August 16, 2013, 08:13:38 AM »

Good luck finding a hot plate these days, especially one that has 240V plug  Grin
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KH2G
Member

Posts: 239




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« Reply #7 on: August 16, 2013, 03:11:49 PM »

light bulbs of proper wattage will work for maintenance purposes and do well with pi and tune-able link outputs. Not so good with a fixed tuned SS rig.
Enjoy,
Dick KH2G
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N1UK
Member

Posts: 1390




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« Reply #8 on: August 16, 2013, 03:48:45 PM »

I use mine all the time. I tune my amp into it, set the cw levels, modulation power levels and rtty power...all OFF the air.

These adjustments are spot on except for my 1/2 wave sloper where the amp will need an on air tweak but the amp dummy load tuning is spot on for my beam, fan dipoles, verticals and 40m 4 Square.

73 Mark N1UK

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KE3WD
Member

Posts: 5694




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« Reply #9 on: August 16, 2013, 04:23:25 PM »

The Dummy Load, along with antenna switch, can and should be used for tuning up amplifiers and the older tube output radios, to avoid those long and annoying tuneups on the air by decreasing the amount of time needed to tune the thing once the antenna is connected. 

73
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N1UK
Member

Posts: 1390




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« Reply #10 on: August 16, 2013, 07:20:41 PM »

I have a nice DX Engineering 8 position antenna switch. The heavy relaybox and coax is on the floor of the shack and the remote control box is on my operating table. It makes it easy and simple to change to a variety of antennas including the oil filled dummy load.


Mark N1UK


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VU2NAN
Member

Posts: 239




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« Reply #11 on: August 17, 2013, 11:21:24 AM »

To W3DBB

Hi OM Doug,

I did that only on 40m.

The impedance was within the range of the Pi tank.

Hence power transfer was not an issue.

The hot plate gave off a glow just as it does on 230V 50Hz!

73,

Nandu.
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KE3WD
Member

Posts: 5694




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« Reply #12 on: August 17, 2013, 03:14:32 PM »

Don't think twice, Nandu, I did it once with one of those Italian oil-filled radiator heaters. 
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VU2NAN
Member

Posts: 239




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« Reply #13 on: August 18, 2013, 04:36:45 AM »

Hi OM Clark,

Many thanks for your encouragement.

73,

Nandu.

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K0JEG
Member

Posts: 638




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« Reply #14 on: August 18, 2013, 06:58:16 PM »

To W3DBB

Hi OM Doug,

I did that only on 40m.

The impedance was within the range of the Pi tank.

Hence power transfer was not an issue.

The hot plate gave off a glow just as it does on 230V 50Hz!

73,

Nandu.

And you can use it to heat up water for tea!
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