flex 6700 swr in 50 Mhz

on5po

Good morning all,
I would like to tell you about a difference "SWR" in 50Mhz, compared to the reading on my flex 6700 and "RigexpertAA520
on flex, 1.7, byp atu and on my rigexpert 1.27
after the flex, I have a SPE amplifier, there I have 1.00 swr, byp atu
after I have swr / watmetre for the 50Mhz reading swr 1.19,
Before opening a ticket, which could give me the explanation
maybe the flex swr / meter is not qualified for the 50 MHz,
If I want to use the flex ATU, the disaster does not seem to work in 50 MHz instead of decreasing, it increases considerably.
Thank you for your opinion
73, ON5PO, Janny

on5po

WK2Y

I have a similar issue with my 6500. I have an LP-700 in line and when the 6500's tuner is in bypass the two SWR's don't match.  I was thinking it was the presence of the coupler and the additional connectors at the high frequency but I don't really know.  Didn't notice until the contest this weekend since I've rarely been on 6. 

Mike-VA3MW

The 6700 has an actual SWR meter in the radio.  Let's see if I can remember this correctly from school.  If I don't have it correct, please take the time to correct me.

Along a 50 ohm feedline unless the Antenna is a 50 ohm resistive load the SWR will vary along the feedline depending on where you measure it and that is based on wavelength.  If you want to do this test, make up a bunch of RF patch cables, 1/4 wave, 1/2 wave and 3/4 wave long and measure the SWR in those points.  That was an experiment I had to do in College and I spent 3 days making patch cables accounting for velocity factor and connectors.  

A simple calibration test is to use either a 25 ohm (2 dummy loads in parallel) or a 100 ohm dummy load and see if you measure 2:1 as that is a calibration point.   If you don't see that on your SWR bridge it out of calibration.

Mike

on5po

Thank you for your feedback ,
I had disconnected, the flex cable and connect it to the rigexpert
now, disconnected, the antenna cable arriving at the shack, connect a load of 50 ohms to it, (welz DL600 swr <1.1 -> 350Mhz)
control with flex swr 2.00 atu byp
with the rigexpert 1.41
spe 1.00 atu byp
swr / watmetre 1.41
you notice that there is still a difference between the flex swr / meter and the expert rig
another control, the dummyload with expert rig -> 1.02 swr
dummyload to flex -> 1.3 swr
I understand that the last check sums up well, the difference of the swr / meter of the flex,
Now the good question, is it possible to calibrate the flex in 50 Mhz?
73 on5po, Janny

Mike-VA3MW

Hi Janny

Can you check both devices on a 25 ohm resistive load please.  You should measure 2:1.

73

HCampbell WB4IVF

“Along a 50 ohm feedline unless the Antenna is a 50 ohm resistive load the SWR will vary along the feedline depending on where you measure it and that is based on wavelength.” 

SWR depends on the load impedance in relation to the characteristic impedance of the transmission line, not on length of the coax.  While impedance does vary with length of the transmission line, the same is not true of SWR (except for small changes due to transmission line loss as the length changes).  This can be shown graphically shown by a constant SWR circle on a Smith Chart. 

Here is a classic article on the subject:

http://www.rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/27%20-%20RF%20FEEDLINE/RF%20Feedline%20-%20My%20Feedline%20Tunes%20My%20Antenna%20%28By%20Byron%20Goodman%20W1DX%20QST%20195603%29.pdf

BTW, the Smith chart is an excellent learning tool for this subject.  There are many tutorials available online.

Howard

on5po

thanks Michael,
Do you still need to have a load of 25 ohms,
I'll look .
thank you,
Now if at flex it is only a bad interpretation of swr, there is no danger, to survive if it does not increase
73

HCampbell WB4IVF

Flex radios decrease output power automatically when reflected power exceeds 25 watts, regardless of SWR, so you are safe. 

Luis del MOlino

When you get different readins on a SWR meter placed in different places on your transmission line (one placed inside the flex and another outside) it is ALWAYS due to RF circulating outside the coaxial cable, because there is no ANY balun at the antenna connection. The SWR readings on a coaxial cable with RF currents well iquilibrated (thans to a balun) if they are identical and opposed  in the central conductor and in the braid , always are the same in ANY PLACE of the cable, because they only deppend of the rate between the impedance Zo of the cable and de Z presented by the antenna.

But this is not true if there is a common mode RF current circulating outside the asimetrical coaxial cable. This common mode current have to be elimnated with a tension  or a current balun.

73 Luis EA3OG

Gary Ashdown

Just a quick note about the 50mhz swr reading on a flex signature seriies 6700. I raised a ticket some time ago about this very issue - end result - the radio has to be returned for realignment.

I tested numerous other radios into the same feedline/antenna (injected at the same point) and only the flex shows a higher swr.

At the end of the day, it was not worth the time or the money to return the radio for this single issue as the swr reading on all hf bands is correct.

regards

Gary

VK8AW