Pages: [1] 2   Go Down

Author Topic: 250W Dry Dummy Load Issue  (Read 488 times)

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
250W Dry Dummy Load Issue
« on: July 13, 2022, 09:37:55 AM »

A few years back I built a 250W dry dummy load in an aluminum box with a finned heat sink.

I used it infrequently, as I also have a Heath Cantenna for up to 500W. It has never had more than 80W FM into it for a few seconds.

Yesterday when checking the output of a transceiver my SWR was 2.80:1 at 2-80W. The antenna reading is usually 1.3 or less depending where I am on the 2M band.

The same cables and switching that goes to the antenna were used for the dummy load. All equipment between the radio and the dummy load was bypassed with the same results. Cantenna has a reading of 1.1:1 at the same outputs.

Dummy load still has the original built resistance of 50.3 ohms.

I'm perplexed.   Any thoughts on why I'm seeing this?
Logged
So Many Toys.......So Little Time!

WA3SKN

  • Member
  • Posts: 8126
Re: 250W Dry Dummy Load Issue
« Reply #1 on: July 13, 2022, 11:01:11 AM »

Well, you could have a bad dummy load, bad meter, and/or faulty cables.
I would look at the cables first... they are the ones that get all the wear and tear, get stepped on, etc.

-Mike.
Logged

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #2 on: July 13, 2022, 11:30:34 AM »

Well, you could have a bad dummy load, bad meter, and/or faulty cables.
I would look at the cables first... they are the ones that get all the wear and tear, get stepped on, etc.

-Mike.

Everything has been checked and verified good.

As per my original post, the same equipment, cables etc into the Cantenna has a 1.1:1 SWR

It's not as you are suggesting.
Logged
So Many Toys.......So Little Time!

W9IQ

  • Member
  • Posts: 8866
Re: 250W Dry Dummy Load Issue
« Reply #3 on: July 13, 2022, 02:12:13 PM »

A few years back I built a 250W dry dummy load in an aluminum box with a finned heat sink.

I used it infrequently, as I also have a Heath Cantenna for up to 500W. It has never had more than 80W FM into it for a few seconds.

Yesterday when checking the output of a transceiver my SWR was 2.80:1 at 2-80W. The antenna reading is usually 1.3 or less depending where I am on the 2M band.

The same cables and switching that goes to the antenna were used for the dummy load. All equipment between the radio and the dummy load was bypassed with the same results. Cantenna has a reading of 1.1:1 at the same outputs.

Dummy load still has the original built resistance of 50.3 ohms.

I'm perplexed.   Any thoughts on why I'm seeing this?

It sounds like you have reactance within the dummy load. This can come from the resistor itself, interconnecting wires, proximity to the cabinet, etc. If you have an antenna analyzer or a VNA, you can verify the presence of reactance.

Other possibilities are a bad solder joint, a poor ground, etc.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W9IQ

  • Member
  • Posts: 8866
Re: 250W Dry Dummy Load Issue
« Reply #4 on: July 13, 2022, 02:39:47 PM »

Just to give you an idea, it would only take about ±30 ohms of reactance to have that SWR if your load is 50 ohms resistive. It doesn't take much in the way of strays on 2 meters to get that reactance.

- Glenn W9IQ
« Last Edit: July 13, 2022, 02:43:23 PM by W9IQ »
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #5 on: July 13, 2022, 02:57:57 PM »

Just to give you an idea, it would only take about ±30 ohms of reactance to have that SWR if your load is 50 ohms resistive. It doesn't take much in the way of strays on 2 meters to get that reactance.

- Glenn W9IQ

Funny thing is that it worked for the last 3 years or so when I used it.

I'll run some test using my VNA and the MFJ antenna analyzer.

Thanks Glenn.
Logged
So Many Toys.......So Little Time!

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #6 on: July 13, 2022, 03:27:59 PM »

Just to give you an idea, it would only take about ±30 ohms of reactance to have that SWR if your load is 50 ohms resistive. It doesn't take much in the way of strays on 2 meters to get that reactance.

- Glenn W9IQ

Funny thing is that it worked for the last 3 years or so when I used it.

I'll run some test using my VNA and the MFJ antenna analyzer.

Thanks Glenn.

In to the Dummy Load Reactance @ 144-148 MHz is 25-30   and lower in the UHF frequencies
SWR is 3.0:1 in VHF and over 4.0:1 in UHF.

All frequencies 54 MHz and lower show R of 42-49 with SWR of 1.0 to 1.2

I am thinking I never used this dummy load for 2M/70cm.

What does peak my curiosity is why the oil filled Cantenna with the large coil resistor does not display these results on the higher frequencies.
« Last Edit: July 13, 2022, 03:35:04 PM by K1KIM »
Logged
So Many Toys.......So Little Time!

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: 250W Dry Dummy Load Issue
« Reply #7 on: July 13, 2022, 07:22:24 PM »

Lead lengths and mounting methods become much more critical as the
load is used at higher frequencies.  Also, the resistor will have a curve
of reactance vs. frequency.

Most dummy loads give a better match the closer to DC they are used,
unless some sort of compensation is added.

What type of resistors did you use?  Many types sold as "non-inductive"
are only good in the lower HF range, but others are good through VHF.

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #8 on: July 14, 2022, 08:27:51 AM »

Lead lengths and mounting methods become much more critical as the
load is used at higher frequencies.  Also, the resistor will have a curve
of reactance vs. frequency.

Most dummy loads give a better match the closer to DC they are used,
unless some sort of compensation is added.

What type of resistors did you use?  Many types sold as "non-inductive"
are only good in the lower HF range, but others are good through VHF.


Load Resistance RFP 250-50 250W 50 Ohm 250N50 TC RF Resistor
Logged
So Many Toys.......So Little Time!

WA3SKN

  • Member
  • Posts: 8126
Re: 250W Dry Dummy Load Issue
« Reply #9 on: July 14, 2022, 09:47:05 AM »

Aluminum is known to have reactive issues with other metals over time.  You might want to just clean/re-solder all connections in the dummy load and re-check.

-Mike.
Logged

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #10 on: July 14, 2022, 10:42:06 AM »

Aluminum is known to have reactive issues with other metals over time.  You might want to just clean/re-solder all connections in the dummy load and re-check.

-Mike.

That is true, but I would think that my resistance would have changed then from the original 50.3 ohms.

I and tending to go along the lines that the resistor is just not applicable for the input frequency of higher than 6 M.

Perhaps, as I mentioned earlier, I was mistaken and never used it on my VHF/UHF rigs.

It still functions perfectly for the lower frequencies.

Thanks everyone for all the input.

Now to just add a label 160-6 ONLY
Logged
So Many Toys.......So Little Time!

AI5BC

  • Posts: 456
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #11 on: July 15, 2022, 04:58:19 PM »


I am thinking I never used this dummy load for 2M/70cm.

You think? Let me guess this dummy load is built with resistors and wires. Then you wonder why it does not work above HF?
Logged

K1KIM

  • Member
  • Posts: 1556
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #12 on: July 15, 2022, 05:41:13 PM »


I am thinking I never used this dummy load for 2M/70cm.

You think? Let me guess this dummy load is built with resistors and wires. Then you wonder why it does not work above HF?

One resistor 1” wires.

Let me guess…… other than sarcasm, do you have anything positive to add??









Logged
So Many Toys.......So Little Time!

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: 250W Dry Dummy Load Issue
« Reply #13 on: July 15, 2022, 07:20:05 PM »

If we assume that the resistor leads are similar to a 600 ohm
transmission line, then 1" will add enough reactance to cause
a 2 : 1 SWR on 2m.  Capacitance to the aluminum box, or
an inappropriate ground path through the case, may increase
that further.

The resistor itself is rated to 3 GHz, but short lead lengths and
careful dressing of the connection to the input coax are required
to obtain the full range.

Also, make sure you don't stress the input tab.  It is designed to
solder to a circuit board of the proper thickness to fit between
the tab and the ground plane.

PA0FRI has some examples on his web site.  Look under MISC
for the article on 150W dummy load.

AI5BC

  • Posts: 456
    • HomeURL
Re: 250W Dry Dummy Load Issue
« Reply #14 on: July 24, 2022, 09:38:31 AM »

One resistor 1” wires.

Let me guess…… other than sarcasm, do you have anything positive to add??
No sir, if you think that will work at VHF and higher, you need to keep playing with CB radios. You clearly do not understand capacitance and inductance, impedance, and resistance.
Logged
Pages: [1] 2   Go Up