Dumb(?) dummy load question

[Eric Ward-3]

In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for two) at the Shack and soldered it to a male BNC--I figured this would make a serviceable QRP dummy load.  The resistor checked out at 50.0 ohm on my DMM.

When I hooked this to my KX1 and went through the ATU alignment procedure (nulling C9, etc), I only get an apparent out put of about 0.3W (according to the display on the radio).  Ditto when I tune my G5RV--I get to 1:1.1, but with only 0.3W.  (This antenna/feedline system is tunable by my LDG Z100 to 1:1; see below.)

I had previously gone through the ATU alignment using, instead of a dummy load, the G5RV routed through the external match (the LDG Z100) tuned for that frequency.  I figured that would  be pretty close to 50 ohm, since it achieves an apparent 1:1 on that freq with my Icom 706 rig.

When I use the tuned G5RV as a "dummy load" and go through alignment, I get what appear to me more reasonable values--i.e. when I subsequently tune the same antenna with the KX1 ATU, I get 4.3W and SWR of 1:1.6.

So something funky is going on and I cannot figure it out.  Is there some bloody obvious reason why the 10W resistor is not a good dummy load?  Is something else wrong with my rig, which otherwise is nominal in all respects?  (BTW, I have R4 maxed out clockwise.)  I do not have a wattmeter, so I cannot independently check output with and without the ATU installed, but the rig is transmitting on 40 and 20m.

Thanks for any ideas...
73,
Eric
N0HHS
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[Mike WA8BXN]

I think those are the wire wound resistors - read that as coil, inductance.
They will be useful as a dummy load only at audio frequencies. The better
way is to get a number of non-inductive resistors and put them in parallel -
20 1K resistors would be 50 ohms, if you use 1/2 watt resistors that would
give you about 10 watts. Such a dummy load will not be prefect, but much
better than a single wire wound resistor!
73/72 - Mike WA8BXN
 
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[David A. Belsley]

Eric:
   I don't know what sort of resistor you got, but many larger wattage
resistors may be fine for 50 ohms at DC, but they can have significant
inductance and capacitance that causes them to have a very different
impedance at RF.  If you have an antenna analyzer like the MFJ 259,
give it a scan.  You are really much better off getting yourself an
honest dummy load designed to be 50 ohms up to 30MHz or better.

best wishes,

dave belsley, w1euy


On Jun 22, 2004, at 9:32 AM, Eric Ward wrote:

> In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for
> two) at the Shack and soldered it to a male BNC--I figured this would
> make a serviceable QRP dummy load.  The resistor checked out at 50.0
> ohm on my DMM.
>
> When I hooked this to my KX1 and went through the ATU alignment
> procedure (nulling C9, etc), I only get an apparent out put of about
> 0.3W (according to the display on the radio).  Ditto when I tune my
> G5RV--I get to 1:1.1, but with only 0.3W.  (This antenna/feedline
> system is tunable by my LDG Z100 to 1:1; see below.)
>
> I had previously gone through the ATU alignment using, instead of a
> dummy load, the G5RV routed through the external match (the LDG Z100)
> tuned for that frequency.  I figured that would  be pretty close to 50
> ohm, since it achieves an apparent 1:1 on that freq with my Icom 706
> rig.
>
> When I use the tuned G5RV as a "dummy load" and go through alignment,
> I get what appear to me more reasonable values--i.e. when I
> subsequently tune the same antenna with the KX1 ATU, I get 4.3W and
> SWR of 1:1.6.
>
> So something funky is going on and I cannot figure it out.  Is there
> some bloody obvious reason why the 10W resistor is not a good dummy
> load?  Is something else wrong with my rig, which otherwise is nominal
> in all respects?  (BTW, I have R4 maxed out clockwise.)  I do not have
> a wattmeter, so I cannot independently check output with and without
> the ATU installed, but the rig is transmitting on 40 and 20m.
>
> Thanks for any ideas...
> 73,
> Eric
> N0HHS
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[Don Wilhelm-2]

Eric and all,

That 50 ohm power resistor you are using will be highly inductive - it is a
wirewound type, meaning it is a coil of resistive wire.  Yes, it measures 50
ohms at DC, but for AC, the resistive (and reactive) value depend on the
frequency of operation.

For those wishing to save a buck on a dummy load, check out the power film
resistors by Caddock (MP series), they are non-inductive up to 500 mHz -
Mouser carries them priced from about $3 to $9 depending on the rating.
They are available in power ratings from 15 to 100 watts, but must be used
with an adequate heatsink - the 15 watt is derated to 1.5 watts without a
heat sink and the 60 watt version is derated to 3.5 watts.  An easy to find
source of heat sink material is computer CPU coolers.  See Tom Hammond's
website www.qsl.net/n0ss for one implementation.  You can see the reisitors
themselves at www.mouser.com/caddock.

Another alternative is the Elecraft DL1 mini-kit - a good 50 ohm load with a
built in tap for connecting the DMM to measure voltage and calculate the
power.

73,
Don W3FPR

Life is what happens when you are making other plans

----- Original Message -----

In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for two)
at the Shack and soldered it to a male BNC--I figured this would make a
serviceable QRP dummy load.  The resistor checked out at 50.0 ohm on my DMM.

When I hooked this to my KX1 and went through the ATU alignment procedure
(nulling C9, etc), I only get an apparent out put of about 0.3W (according
to the display on the radio).  Ditto when I tune my G5RV--I get to 1:1.1,
but with only 0.3W.  (This antenna/feedline system is tunable by my LDG Z100
to 1:1; see below.)



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[Don Nesbitt]

Hi Eric - if the 10W 50 ohm resistor is NOT a "non-inductive" resistor (and
if you got it at RS it is a pretty sure bet that it is a wire-wound - and -
they are not "non-inductive - but - being wire-wound - are very
inductive!!), then you are having problems because it is NOT presenting a
pure (or reasonably pure) 50 ohm load to your KX1 even though the resistor
reads 50 ohms at DC (as read by your DMM).  At RF frequencies, it will
exhibit "reactance" in addition to the "resistance" and this will make it
look like something other than a resistive dummy load to the KX1.  It will
look more like a pretty poor antenna with very high SWR and the readings
that you get on the KX1 in the calibrate position will be incorrect - VERY
incorrect!!

Most likely this is what you are experiencing and there is nothing wrong
with the rig as evidenced by the fact that when you run it into an SWR close
to 1:1 it seems to read correctly!

Bottom line - see if you can get your $ back at RS and get an actual dummy
load that is close to purely resistive at RF.  Have fun es 73  --don n4hh
K2/100/KAT100/etc/etc/#2028 and KX1/etc #243

----- Original Message -----
From: "Eric Ward" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, June 22, 2004 9:32 AM
Subject: [Elecraft] Dumb(?) dummy load question


In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for two)
at the Shack and soldered it to a male BNC--I figured this would make a
serviceable QRP dummy load.  The resistor checked out at 50.0 ohm on my DMM.

When I hooked this to my KX1 and went through the ATU alignment procedure
(nulling C9, etc), I only get an apparent out put of about 0.3W (according
to the display on the radio).  Ditto when I tune my G5RV--I get to 1:1.1,
but with only 0.3W.  (This antenna/feedline system is tunable by my LDG Z100
to 1:1; see below.)

I had previously gone through the ATU alignment using, instead of a dummy
load, the G5RV routed through the external match (the LDG Z100) tuned for
that frequency.  I figured that would  be pretty close to 50 ohm, since it
achieves an apparent 1:1 on that freq with my Icom 706 rig.

When I use the tuned G5RV as a "dummy load" and go through alignment, I get
what appear to me more reasonable values--i.e. when I subsequently tune the
same antenna with the KX1 ATU, I get 4.3W and SWR of 1:1.6.

So something funky is going on and I cannot figure it out.  Is there some
bloody obvious reason why the 10W resistor is not a good dummy load?  Is
something else wrong with my rig, which otherwise is nominal in all
respects?  (BTW, I have R4 maxed out clockwise.)  I do not have a wattmeter,
so I cannot independently check output with and without the ATU installed,
but the rig is transmitting on 40 and 20m.

Thanks for any ideas...
73,
Eric
N0HHS
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[Jeff Anderson-5]

Hi Eric,

I tried this once.  The 10W 50 ohm resistors that I purchased at RS are
wirewound, and make a terrible load at RF frequencies.  The higher the freq,
the worse they get.

For reference, I just grabbed one of my resistors, and made a few
measurements with the MFJ antenna analyzer:
3.75 MHz measures as 1.9 SWR
7.12 MHz measures as 3.2 SWR
14.1 MHz measures as 7.6 SWR

Your G5RV through a tuner probably IS closer to 50 ohms, but some
combination of carbon resistors (for a total of 50 ohms) would probably be
even better.

73,
Jeff Anderson  KD7PAW

----- Original Message -----
From: "Eric Ward" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, June 22, 2004 7:32 AM
Subject: [Elecraft] Dumb(?) dummy load question


In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for two)
at the Shack and soldered it to a male BNC--I figured this would make a
serviceable QRP dummy load.  The resistor checked out at 50.0 ohm on my DMM.

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[Bob Nielsen]

On Tue, Jun 22, 2004 at 09:32:30AM -0400, Eric Ward wrote:

> In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for two) at the Shack and soldered it to a male BNC--I figured this would make a serviceable QRP dummy load.  The resistor checked out at 50.0 ohm on my DMM.
>
> When I hooked this to my KX1 and went through the ATU alignment procedure (nulling C9, etc), I only get an apparent out put of about 0.3W (according to the display on the radio).  Ditto when I tune my G5RV--I get to 1:1.1, but with only 0.3W.  (This antenna/feedline system is tunable by my LDG Z100 to 1:1; see below.)
>
> I had previously gone through the ATU alignment using, instead of a dummy load, the G5RV routed through the external match (the LDG Z100) tuned for that frequency.  I figured that would  be pretty close to 50 ohm, since it achieves an apparent 1:1 on that freq with my Icom 706 rig.
>
> When I use the tuned G5RV as a "dummy load" and go through alignment, I get what appear to me more reasonable values--i.e. when I subsequently tune the same antenna with the KX1 ATU, I get 4.3W and SWR of 1:1.6.
>
> So something funky is going on and I cannot figure it out.  Is there some bloody obvious reason why the 10W resistor is not a good dummy load?  Is something else wrong with my rig, which otherwise is nominal in all respects?  (BTW, I have R4 maxed out clockwise.)  I do not have a wattmeter, so I cannot independently check output with and without the ATU installed, but the rig is transmitting on 40 and 20m.
>
> Thanks for any ideas...

Is it a non-inductive resistor?  If not, it probably won't be anywhere
near 50 ohms at R.F.

Bob, N7XY
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[Eric Ward-3]

Many thanks to all the replies indicating correctly that the resistor I was
using is wirewound, a.k.a. a coil/inductor at RF, and therefore not a useful
dummy load.
Live and learn...
73,
Eric
N0HHS

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[Trev - K6ESE]

Hey Gang,

This brings up a question I've wondered about. Are Metal Oxide power
resistors inductive? Such as these:
http://www.xicon-passive.com/specs/mo.pdf ? I don't see a spec in the data
sheet.

73's Trev - K6ESE
http://www.qsl.net/k6ese


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[Don Brown-4]

Hi

MOS resistors are not inductive at HF frequencies and make great dummy loads. The Elecraft DL1 and the OHR 100 watt dummy load use several MOS resistors in parallel or series parallel. The oxide is deposited on a ceramic base in a single coating so there is not any coiling to create inductance. I use them to make dummy loads by soldering a 51 ohm 5 watt inside a PL259 connector. You can use a PL259 to BNC adaptor so it will fit the K2, K1 or KX1. These loads will take 15 watts for a short time. I also have one of these with a 100 ohm MOS resistor and another with a 25 ohm resistor (2 51 ohm in parallel) that I use to check or calibrate the SWR bridges in the antenna tuners or KPA100. You can get them from Mouser in 1 watt 3 watt and 5 watt sizes

Don Brown
KD5NDB


  ----- Original Message -----
  From: Trevor Jacobs<mailto:[hidden email]>
  To: [hidden email]<mailto:[hidden email]>
  Sent: Tuesday, June 22, 2004 12:35 PM
  Subject: [Elecraft] Metal Oxide Power Resistors Inductive?


  Hey Gang,

  This brings up a question I've wondered about. Are Metal Oxide power
  resistors inductive? Such as these:
  http://www.xicon-passive.com/specs/mo.pdf<http://www.xicon-passive.com/specs/mo.pdf> ? I don't see a spec in the data
  sheet.

  73's Trev - K6ESE
  http://www.qsl.net/k6ese<http://www.qsl.net/k6ese>


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[Stuart Rohre]

Eric, the RS resistors are inductive, and probably wire wound and unsuited
for 50 ohm impedance to RF.   You would need compensation capacitor networks
for them for each band.

You only measured their DC values as 50 ohms.  Impedance is an AC /RF
quantity.

RS used to carry a true QRP dummy load, black with fins and coax connector.
MJF and Ten Tech sell low cost non inductive dry resistor loads.  Ten Tec's
is a kit.
73,
Stuart
k5KVH


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[Stuart Rohre]

Trev,
Way back in 1960's  I wondered the same about Corning tin oxide resistors
like these axials.

I measured many after work with HP Vector Impedance Meters, forerunners to
the MFJ et al.

The results I got showed they were good up to 108 MHz the high end of the
4815 Z meter.  I also had run them down below 500kHz with the 4800 Z meter.

It is likely these are OK, depending on how they are deposited.  If spirals
are done solenoid style there is likely some inductance, but if wound right,
it can be canceled.
72,
Stuart
K5KVH


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