Dummy Load showing 1.7 SWR

[WRUT935]

I just for this 50W 50 ohm dummy load.  XRDS-RF from Amazon.  I directly connected it to my SureCom meter, no cables and no adapter needed. I keyed up and read both wattage and SWR, the  SWR shows 1.7 which is higher than what I get through my outside antenna with 45 feet of cable!  Is this just a bad dummy load or am I missing something.  I expect 1.1.  I measured the resistance using a digital ohm meter and it read 49 ohms.  

Why am I checking SWR?  To use the dummy load to measure line loss of the cable.  Professor Ed Fong told me it would be  good reference to measure the SWR introduced by the cable and measure again with the antenna.  This way, you can tell what is contributing to the overall value.

I know there are a lot pricier dummy loads, is this brand/unit bad?

 

[SteveShannon]

6 minutes ago, WRUT935 said:

I just for this 50W 50 ohm dummy load.  XRDS-RF from Amazon.  I directly connected it to my SureCom meter, no cables and no adapter needed. I keyed up and read both wattage and SWR, the  SWR shows 1.7 which is higher than what I get through my outside antenna with 45 feet of cable!  Is this just a bad dummy load or am I missing something.  I expect 1.1.  I measured the resistance using a digital ohm meter and it read 49 ohms.  

Why am I checking SWR?  To use the dummy load to measure line loss of the cable.  Professor Ed Fong told me it would be  good reference to measure the SWR introduced by the cable and measure again with the antenna.  This way, you can tell what is contributing to the overall value.

I know there are a lot pricier dummy loads, is this brand/unit bad?

 

A purely resistive (no reactance) 50 ohm  dummy load should give you a 1.0:1 SWR, meaning all the power is forward and none is reflected. It should read 50 ohms, not 49. 
But it’s difficult to say whether it’s your dummy load or your Surecom meter.   Or both. 

[marcspaz]

I have an XDRS dummy load and an attenuator.  They are cheap, but not bad. 

 

Without seeing anything first hand... I would say that the transmission line on the antenna is balancing the load.  Additionally, I would never use a 50w dummy load to test a 50w radio.  I would use a minimum of 100w, but 300w would be better.  This is because of the duty cycle. My 100w XRDS-RF directions say that at 100w, the dummy load can only be used for 10 seconds, and then it needs to be allowed to cool for an additional 600 seconds. 

That said, if you used it for more that 10 second in a 10 minute time period, you probably cooked the resistor enough that the resistive value changed.

Also, Surecom SWR meters tend to be pretty close out of the box, but they are user configurable.  Sometimes, they do need to be calibrated.

In any case, if the meter is "close enough", chances are the SWR on the dummy load is low enough that its safe for the test you want to run.  Though, me personally, I would be more prone to use a real and calibrated field strength meter and compare that to a custom tuned dipole, since that is the benchmark that gain is typically measured against.  A few $$$ and some wire, you can make a dipole for under $3, but the meter may cost $100 or more for a decent one that is "close enough".

[Lscott]

25 minutes ago, WRUT935 said:

I just for this 50W 50 ohm dummy load.  XRDS-RF from Amazon.  I directly connected it to my SureCom meter, no cables and no adapter needed. I keyed up and read both wattage and SWR, the  SWR shows 1.7 which is higher than what I get through my outside antenna with 45 feet of cable!  Is this just a bad dummy load or am I missing something.  I expect 1.1.  I measured the resistance using a digital ohm meter and it read 49 ohms.  

Why am I checking SWR?  To use the dummy load to measure line loss of the cable.  Professor Ed Fong told me it would be  good reference to measure the SWR introduced by the cable and measure again with the antenna.  This way, you can tell what is contributing to the overall value.

I know there are a lot pricier dummy loads, is this brand/unit bad?

 

I'm guessing the dummy load looks something like this?

https://www.amazon.com/XRDS-RF-PL259-Male-RF-Dummy/dp/B08QR9RQJ3/ref=sr_1_13?keywords=XRDS-RF&qid=1678988619&sr=8-13

I wouldn't worry about the resistance reading of 49 verses 50 ohms. There is a tolerance on the accuracy of the dummy load and the meter itself that likely accounts for the difference.

Dummy loads also have a frequency range they are designed to work over. High quality ones may even have a datasheet showing the expected match verses frequency. The one you have may just be at the edge of the range, but without further info I can't really say much at that aspect. 

The Surecom meters people have reported funny behavior. One of them is a change in SWR with a change in power level. Theoretically the SWR should not change when the power level changes.

The other thing with SWR has to do with the coax cable. A longer cable will absorb more of the reflected power from the antenna end back to the transmitter. With the meter at the transmitter end this makes the SWR look better compared to making the measurement right at the antenna. I've taken advantage of this to get a match under 2:1 on a radio when using an antenna outside of it's specifications and the loss in power, on the coax to the antenna, is the price paid for the benefit. There is no free lunch.