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Author Topic: 50 Ohm Dummy Load  (Read 2902 times)
W1KQ
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« on: September 10, 2009, 05:14:15 PM »

Please explain for
the benefit of those
who don't know why a
50 ohm VHF dummy
load is not
necessarily 50 ohms
at HF.  Especially
for the benefit of
someone who believes
a 50 ohm resistor is
a 50 ohm resistor
regardless of fo.
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KZ1X
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Posts: 3227




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« Reply #1 on: September 10, 2009, 05:18:35 PM »

Actually, I rather imagine a 50 ohm nonreactive load at VHF is probably also nonreactive at HF.  It's the other way around that isn't often so.
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WB2WIK
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« Reply #2 on: September 10, 2009, 05:20:43 PM »

I agree with Steve KZ1X.

A good 50 Ohm resistor is 50 Ohms from  DC to whatever its upper frequency limit is, determined by reactive components that start to impact its impedance.

I've never seen a "VHF" dummy load (that actually worked at VHF) that wasn't an even better dummy load at HF.

WB2WIK/6
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AA4PB
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« Reply #3 on: September 10, 2009, 05:25:04 PM »

A good 50 ohm VHF dummy load will be 50 ohms all the way down to DC so it will work fine at HF.

Now the resistors used in dummy loads can have some inductance in addition to the resistance and that can cause the impedance to be something other than the required 50 ohms, no reactance (50-j0). Special non-inductive resistors are normally used for the larger loads but they are not perfect. Usually, however, the inductance causes more problem at VHF than at HF so you often find loads that work fine at HF but don't work well at VHF and above.
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W1KQ
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« Reply #4 on: September 10, 2009, 05:27:29 PM »

C'mon guys, I'm kind of looking for some support here.  Had someone who couldn't get their rig's auto-tuner to load into a Radio Shack dummy load (supposedly for VHF-per this individual) but my dummy load worked perfectly with the same radio.  I suspect their 50 ohm load wasn't non-reactive.  May I say key word: Radio Shack?
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KE4DRN
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« Reply #5 on: September 10, 2009, 05:31:05 PM »

hi,

perhaps the RS load was damaged by excessive power ?

73 james
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WB2WIK
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« Reply #6 on: September 10, 2009, 05:32:51 PM »

Can you say "defective?"  

Which would apply to a lot of stuff from Radio Shack!

Best 50 Ohm dummy load for VHF, unless you have a Bird Termaline or equivalent, is about 1000' of RG58/U, whether anything's connected to the far end, or not.

;-)
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WB6BYU
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« Reply #7 on: September 11, 2009, 08:02:53 AM »

Was it a dummy load designed for CB?  The kind built into
a PL-259?  They are rated at 5W max and can easily be
smoked by the full output of a transceiver.

If the problem is that the tuner can't match it, the
dummy load must be WAY off from 50 ohms - I'd suspect that
if you check it with an ohmmeter you'd see that something
was wrong.  This isn't the sort of thing that would be
caused by the difference between HF and VHF dummy loads.
(Unless the dummy load was intentionally built to have
a limited operating range.
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KF5AEO
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« Reply #8 on: September 11, 2009, 08:25:32 AM »

i was once told a 150 watt lightbuld makes a great dummy load    how true is this?





dummyload= grab the antenna while i key up   hihi
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N1DVJ
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« Reply #9 on: September 11, 2009, 09:53:04 AM »

USUALLY a dummy load will fall out of 'compliance' as the frequency goes up, not down.  A dummy load for HF will start acting flakey as the frequency goes up and all kinds of parasitics and internal 'leakages' come in to play.  

In the other direction, there's nothing to prevent a VHF or higher dummy load from working at lower frequencies, other than power ratings.  And that should be fairly obvious.

On the other hand, it could be something built so bad that it DEPENDS on stray leakage and coupling to work (especially if it's a load built 'in' to a meter or other piece of test gear for VHF) and when you try to operate it out of it's normal frequency area, you lose the interaction to the 'stuff' around it and now it appears to be bad.

Most of your 'CB' type dummy loads are actually a 2W resistor inside a PL-259 plug with a plastic pipe cap on the end.  They depend on the user not cooking the thing for extended periods to keep from popping the resistor.

Radio Shack actually sold a really great VHF 25W dummy load (contrary to the no-nothings that think that all RS stuff is junk.  But then, consider the source!)  but it was only sold for a short time and wasn't in all stores.  Now I think they don't even carry a CB dummy load any more.

You can make your own fairly easily.  Just get a 50ohm NON-INDUCTIVE resistor of at least 2W rating and solder it into a PL259.  For intermittant duty, it should be good for 4 or 5 watt, and probably up through VHF.

I've made my own for years.  But I also make a 75ohm (gives me a known 1.5:1 SWR) and a 100 ohm (a known 2:1) for when I'm testing.  (In the CB craze days, you could also use the 100ohm load to help you tune up any of the 'co-phased' antennas truckers used to love.)

Avoid using any of the BNC or F type dummy loads.  They are for small signal work, and generally have power ratings in the fractions of a watt.

Use a light bulb?  Well, I know a lot of hams that used to do that years ago.  In fact, I used to do that in college.  But it's more of a 'sorta work' kind of thing than what you would really want to do.  However...  some habits just won't die.  There are groups that now run 'light bulb' contest to make contacts from your 'dummy load light bulb'.  Some of the rules limit feed line length and position (gotta be 14/2 if I remember) and you gotta use a ceramic screw type base like you find in old basement ceilings.  Kinda fun, but not too many of the transistor rig type guys like it.  I think for best performance, the wattage of your bulb should match your output, or be a little higher.  But then, I don't really know.  Just realize when your bulb is off and cold, it's gonna look like close to a short to your rig!
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WB2WIK
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« Reply #10 on: September 11, 2009, 10:00:49 AM »

>RE: 50 Ohm Dummy Load       Reply
by KF5AEO on September 11, 2009    Mail this to a friend!
i was once told a 150 watt lightbuld makes a great dummy load how true is this?<

::Not very.  Lighbulbs make poor dummy loads, although back in the 40s-50s-60s it wasn't so uncommon to use one (I did) as they're cheap and provide a "visual" confirmation that you actually have transmitter output power!  But they're lousy because:

-They're sure not 50 Ohms!

-Their impedance changes with power applied and filament temperature.

-They radiate through the glass, and 100-mile contacts can easily be made using a light bulb for a dummy load, which kind of defeats the purpose of using a dummy load.

It's a fun experiment, but not a serious load.

WB2WIK/6
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AA4PB
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« Reply #11 on: September 11, 2009, 10:02:08 AM »

i was once told a 150 watt lightbuld makes a great dummy load how true is this?
------------------------------------------------------
No, it doesn't make a "great" dummy load, but it can sometimes be used when nothing else is available. It's not 50 ohms, it's not non-inductive, and the impedance changes with temperature as it lights up.

In the tube days it could be used because the pi networks had a pretty wide matching range and we didn't have things like SWR foldback circuits in the transmitters. Today it can be a decent test for a wide range automatic tuner.
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AA4PB
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« Reply #12 on: September 11, 2009, 10:04:04 AM »

Back in the 50's I used to use a #47 pilot lamp bulb as a dummy load to check for output on a QRP rig.
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AA4PB
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« Reply #13 on: September 11, 2009, 10:08:59 AM »

I'm kind of looking for some support here...
-----------------------------------------------------
It's probably not because the load worked at VHF and not at HF. It probably didn't work on VHF either. If it has inductive reactance on HF then it'll have even more inductive reactance on VHF.

I didn't think Radio Shack sold any "VHF" dummy loads - most everything was for 27MHz. In fact, I don't think I've ever seen a dummy load rated for VHF only.
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