On 30m the Flex reads 1.3:1 (hard to be exact due to poor scale, it needs numbers). However, the external DN-801 reads just over 2.1:1.
Without having to buy yet another meter, any ideas?
SWR is NOT a function of transmission line length. The length of coax can act as an impedance transformer to a degree but has nothing to do with the impedance of the antenna which causes the line reflections. Unreliable SWR readings usually have something to do with RF flowing on the outside of the transmission line, and poor grounding.
Agree with Jim. If the radio has a problem finding a match, changing coax length can change the impedance (but not actual SWR) presented to the radio, and can result in the radio finding a match. But the radio finding a match does not seem to be a problem in this case. If changing coax length significantly changes the SWR reading, I’d suspect RF on the shield.
And keep in mind that a meters’ calibration (especially a modest one like the Daiwa) can vary significantly with frequency and power. If I had to choose one I’d go with the Flex.
BTW, I notice similar behavior with the modest LDG M-1000 meter connected to my LDG-1000 tuner. At times it indicates significantly different (usually higher) SWR than my Flex and LP-700.
As you might guess, I have nothing better to do than write responses to this post. It's bad weather here and everything is covered with ice.
Pretty much everything said here is true, but does not necessarily address Steve's question.
To answer your question, don't buy another SWR meter, sell the Daiwa. Then you will only have the Flex indicator and that's the only one that matters.
The only reason to be concerned about high SWR at all is the effect the reflected power has on the radio's output stage. I do not know anything about the Flex output stage, but the general effect of high reflected power is to raise the voltages on the output transistors and raise the heat dissipation. At some point the transistors burn out.
With a SWR of 1.3:1 as indicated on the Flex, that is not a problem in your case.
One response from Jim, K6QE mentions current on the outside of the coax shield as a possible cause of different instruments reading different SWR. This is called common mode current and is a very likely cause of reading differences - it does not affect the actual reflected power the radio is seeing.
An easy test for CM current is to connect an antenna analyzer in place of your radio and set it to indicate the SWR at the frequency of interest. If the SWR seems to change when you touch the analyzer or the connections with you hand, you have common mode current.
The ARRL Handbook lists several easy fixes for CM current and I have found them to be effective.
I second the opinions expressed that the meter in the Flex is very accurate. Matches the reading of the LP-100 very well.
I don't want to throw away power either and losing 25 watts out of 100 w represents a 1.25 dB loss of signal strength at the far end excluding any cable loss. And you are right, the transmitter will reduce its output to prevent damage due to excessive reflected power. This "fold back" does not change the SWR, it just reduces incident power which in turn reduces reflected power in proportion. Either way you look at it, the power received at the distant end is reduced.
But that was not my point. Without knowing more detail of the Flex output stage, it's hard to say what happens to the reflected power. It is entirely possible that most of it gets radiated.
If the output impedance of the Flex is 50 ohms - a match to the coax - then that's the worst case. The reflected power comes down the coax and is absorbed by the Flex output stage. That's bad and that's why the Flex acts to reduce reflected power by reducing incident power.
On the other hand, if the output impedance of the transmitter is lower than the coax characteristic impedance, say 5 or 10 ohms, the reflected power sees it as a mismatch and much of it is re-reflected back towards the antenna. In that case, most of the power eventually gets radiated.
A lot of considerations go into the design of an amplifier but in my case, I have always strived for a low output impedance. To launch 100w up a 50 ohm coax, all you have to do is have an output stage capable of providing about 71 volts at 1.4 amps at the same time and you're good to go. The amp's output impedance is not all that important but, If it is low enough, you don't have to worry (much) about reflected power because it will be reflected back towards the antenna.
I'm not saying high SWR or more specifically high reflected power is good, it isn't but it is not always what it seems and it's not always a total loss. In Steve's case, an SWR of 1.3:1 as measured by the Flex gives only about 0.2dB of signal loss if every bit of the reflected power is absorbed and never seen again.
I agree with the other writers that every SWR meter (excluding lab instruments) seems to give a different value. I guess you get s much precision as you are willing to pay for.
- Nothing will correct the intrinsic impedance of the coax with the antenna impedance short of correcting the antenna, but that is not as important as originally thought.
- No-one mentioned the other important item, being the loss per 100ft of the coax
- The best place for impedance matching is at the antenna end of the coax, but that may not be the most convenient position for that matching box
- A portion of the reflected power (wherever the matching box is located) will be re-reflected back towards the antenna, with additional associated coax power loss. And a portion of that re-reflected power will go out the antenna except for very little re-re-reflected power.
- Impedance presented to the transmitter will be transformed by additional length of coax, or not transformed as much if a length is removed. All depending on the wavelength being added/removed (also considering the velocity factor of the coax)
- Reflected power (that passes through the matching box) back to the transmitter will be dissipated in the final amplifier as heat. That is why high power linear amps are de-rated when SWR is high.
SWR meters use a short length of coax with an embedded extra wire that picks up a directional current. That current will be affected by where the meter is positioned.
There is NO antenna mismatch! The antenna is a 3 element steppir with 40m trombone section added. It has nearly perfect SWR on every band and re-tuned per 100kHz to track the TCVR frequency. I have checked this fact with a MFJ antenna tuner to be correct on all bands including 30m.
The only differences that might explain the anomaly is just 12 inches of coax between the radio and the swr meter. After that point everything else is the same. 12 inches is way to short as a function of 30m band to be significant but I will replace it just to be sure.
If the DN-801 meter where bad I'd expect it on all bands, or a worsening as I moved HF, which does not happen.