Tuner issue on 80m

Alan - KA4B

I have a Flex 6500, which I am very happy with, but from the beginning I have had a tuner issue at about 3.6 Mhz.  The autotuner will simply not tune between about 3.575 and 3.625.  The measured SWR is about 3.14.  I sent it back to Flex more than a year ago for checking and they didn't find any issues.  Steve tells me that it should tune anything under an SWR of 10, and it appears to do so everywhere else, including 160 M for which I do not even have an antenna.  I am using an 80 meter dipole resonant at about 3.8 Mhz with a 40 meter dipole fed from the same point drooping about 5 feet below it, with a balun.  It tunes fine everywhere on 40 and everywhere else but around 3.6 on 80.  Changing the coax length doe not seem to affect this.  Any one else had a similar issue?  Any ideas?

Jim Gilliam

Put a lumped circuit on the output of the transceiver than would simulate a 4:1 SWR like a 12.5 Ohm resistor and see what happens.

Jim, K6QE

Alan - KA4B

Jim, there are other places in the 80 meter band where the SWR is higher, such as at 3.51 MHz, where the tuner handles it fine.  And of course the SWR is much higher on 160 and it works.  My original theory was that there was a bad relay to one of the inductors or capacitors in the tuner, but Flex says that is not the case.

George KF2T

Your SWR value may be misleading, with your antenna presenting a complex impedance outside the range of the tuner. Coupling between the two antennas, feedline effects, and even station grounding can affect what the rig sees. Have you looked at it with a good analyzer or impedance bridge? If it is possible, place the 40 meter legs 90 degrees from the 80m legs to reduce interactions.

KF4HR

No such issue with my 6700.  I'd be curious to know if you get the same tuning issue at that frequency, with a 50 ohm dummy load.

Jay -- N0FB

My guess is that the required inductance to match at that frequency is just more than the internal tuner can handle.

Alan - KA4B

With the dummy load the tuner drops out immediately, because it is not needed, no matter what frequency I have the radio set on.  On my wideband folded dipole it tunes completely across the band.  It even works when tuning 80 on my hex beam. On 80 meters it tends to grind and grind much longer than it does on any of the other bands.

Alan - KA4B

George, going at 90 degrees with the 40 is not practical based upon where the trees are located, but I suppose I could lower the 40 meter antenna way down and see if it changes.

Alan - KA4B

I dropped the 40 meter antenna about 50 degrees on both ends.  The situation remains pretty much the same.

Alan - KA4B

Are you saying that could be the case even though the tuner is supposed to be able to handle 10:1 and it is just slightly over 3:1?  Does your tuner ever grind for 8 to 10 seconds?

W7NGA

The impedance is complex .. I run extended-double-zepps exclusively and many times the SWR is relatively high (not for open-wire feed) but not low enough to not require a complex (pun intended) conjugate solution. The tuner just doesn't have the range and that is acceptable. Fringe solutions generally require an external tuner, especially due to interactions that may exist at full power but are benign at tune levels.

W7NGA  dan
Seaside, Oregon

Alan - KA4B

The only thing I can think of that might be a potential cause is that the RG 213/U I am using is about 16 years old.  Perhaps I should just replace it and see if it makes any difference.

Mark Erbaugh

I've had similar problems with the tuner on my 6700 when I was using a Carolina 160 (OCF) on 80. There were places that other tuners would handle with ease, but the tuner in the Flex would grind and grind (the relay clicking really sounds like grinding) and then give up.  I no longer have that antenna up, but as I recall, the grinding started with one rhythm and after a few seconds if it didn't find a match went to a second rhythm. If it went to the second rhythm, it would never find a match. At some point during the tuning process, the indicated power dropped to zero. I didn't have an external watt meter so I don't know what the actual power out was.  The odd thing was that I could find spots where it wouldn't find a match but 25 kHz above and below it would find matches.

Jay -- N0FB

There are a lot of ways a 10:1 or 3:1 SWR can present itself.  Most internal tuners are able to add larger resistive loads or differences than a match which needs high reactance.  Because an internal tuner has such severe size constraints, you don't have the space for large coils to add larger amounts of inductance.  This is the very reasons I use an external antenna tuner.

HCampbell WB4IVF

George is right, autotuner SWR specs can be misleading.  For example, my LDG-1000ProII is “spec’d” to handle a 10:1 SWR, yet on 160M it will not tune the top 20kHZ of the band, even though the SWR is in the 3:1 range.   The LDG just can’t match the complex impedance values presented by the simple dipole antenna in this part of the band, even though the SWR is supposedly well within its “range”.   I’ve run across similar situations with other autotuners.  My “solution” was to connect the LDG and a manual tuner is series, with the manual tuner usually in bypass mode.  When the autotuner refuses to cooperate, I put it in bypass and use the manual tuner. 

Mike W1BFA in Maine

This probably doesn't help much Alan, but for the heck of it this morning I tried the same freqs on my 6500 with my normal 80 mtr ant. ( a full wave 160 horz loop fed corner with open wire ) and saw similar results.  I then switched over to my 5000a and did the same test and got some of the same effect although the 5000 is somewhat more forgiving.  This got me curious so I put the manual tuner on and it tunes those freq fine but you have to crank in quite a bit of inductance.  The big loop is a good antenna but it's got some odd points where it really presents a poor match without some inductance.  I'm an antenna hack so I can't really explain the reasons behind it although I sometimes think it has some relationship to feed line lengths.

just tried my little Yaesu 450 with the LDG450 tuner and its similar also...   waters muddy enough for you?    Good luck!  Mike

Bill W2PKY

Try adding an additional length of feed line until it has success. I Use a 160M inv "L" on all bands but needed to add some coax to get the tuner to handle some bands.

Mike W1BFA in Maine

Hi Bill..  on my antenna I have about 40' of open wire to a 50' section of 50 ohm coax.  For the heck of it today I will put in another section of coax and see what difference it makes.  I had never noticed the "notch" of poor tuning on 80 till I saw Alan's post.  If I'm in that area usually use the amp and external tuner.  Does this mean its almost antenna season?  :-)   usually in Maine we wait till there's snow on the ground and you can't feel you fingers unless you hit them with the torch..

W7NGA

Hams seemingly have no issue with paying $300 for a set of earphones or code-key whereas an antenna analyzer for the same cost is the better investment. It would certainly facilitate a better understanding of the antenna and the mysteries therein.

Alan - KA4B

Dan, I have characterized my SWR on all bands using the Flex Meter software.  I see from QRZ that you appear to be very much into test instruments.  What would I expect to learn from an antenna analyzer that I do not already have access to?   

Rick WN2C

I thought the Flex internal tuner could only handle an approximate 3:1 swr? Does it really handle a 10:1 ? There are not many internal tuners that can do that.

W9OY

Stick 30ft of coax in the line and see what happens

Alan - KA4B

Rick, that is what Steve Hicks told me at the Huntsville Hamfest a couple of weeks ago.  The actual specs say 8.3 to 300 Ohms, which would be 6:1 on either side.  

Alan - KA4B

Magic!  Seriously though, I added a 24 foot piece that I had on hand.  It now tunes throughout the 80 meter band.  I tested every 25 kHz.  The worst match is about 1.35.  There are three dips to 1.16 or below.  I have read about this in the ARRL Handbook but have never really been able to make sense of it.  What is really happening, and am I just hiding a worse SWR problem at the antenna?  Obviously I am increasing my losses and perhaps I am causing new problems on other bands.

W1RE

The feedline is an integral part of the load seen by the 6500 and is every bit a part of the circuit as the elevated dipole sections. Changing the length of the feedline alters the impedance the lump circuit presents to the internal tuner and therefore alters the ability of the internal tuner to match.

Bob

W9OY

A "feedline" presents a constant impedance only at 1:1 match.  At a match where there is SWR (not 1:1) the feedline acts like a stub.  The impedance presented on the end of a stub goes toward open (1000's of ohms) or towards short (zero ohms) as you vary frequency and those extremes can often be quite sharp (like 50khz) when they appear.   Either one of these extremes move you out of range of the tuner.  30ft (or 24ft) is approximately 1/8 wavelength on 80M so it changes the occurrence of the open or short condition to somewhere outside the 80M band so the impedance becomes something the tuner can match.   the extra length does not increase your losses by any real extent (maybe a tenth of a dB)

73  W9OY

Alan - KA4B

So the 'open' or 'short' condition would not be seen as a spike in SWR?  And if not, should you look for those frequencies with an antenna analyzer, so as to avoid them in the bands of interest?  I have some squirrel nibbles in the RG 213/U, (which has a solid dielectric,) that I sealed up.  As long as they did not create a physical discontinuity or short, could that affect my ability to tune? BTW, I am thinking that considering the velocity factor of 0.66 1/8 wavelength would be about 22 feet.

W9OY

In this case the spike in the SWR is on the transmitter end.  The antenna impedance is set by it's length and the frequency

Squirrel nibbles might allow water ingress, but probably unrelated since the 24ft worked.  Yes you do take into account velocity factor.  My coax is about .8 so that's why I cut down to around 30ft.  You can get all **** about analyzing this, but just remember 1/8 eave plus or minus and you will be doing it the ham's way

73

Alan - KA4B

Lee, I have tried to find a technical description of this in the internet and in the ARRL Handbook.  The following website: http://www.hamuniverse.com/feedlinelengths.html tends to agree with what you said, but describes using this method for feed lines of 300 to 600 ohms.  Another website says to cut the coax to .25, .5, .75 or 1 wavelength for coax.  These are clearly contradictory to each other.  Of course both of these need to be adjusted for the velocity factor of the coax.  In my case my adjusted original length is 0.83 wavelength, which should have been OK according to your recommendation and that of the website above.  I do not like the cut and try approach.  I will probably buy a new piece of coax and start over, but I would like to be sure that I am cutting it the right length to begin with.  Can you refer me to a good reliable article somewhere that deals with this subject?  Thanks!

Luis del MOlino

This is a clear syntom that you do not have a balun in the center of your 80 meter dipole and you have some RF ciriculating outside the braid of your coaxial line. The SWR you are seeing is a mixture of the SWR insede of the coaxial and the impedance seen between the outside braid and the center conductor. It is not the real one. 

All your troubles will dissappear if you put a balun (tension or current) in the feeding point of your 80 meters antena and another Unun before your rig.

Try it.

Luis EA3OG

Alan - KA4B

Luis, I mentioned in the original post that I have a balun.  It is this one: http://www.balundesigns.com/model-1116d-1-31-mhz-5kw-optimized-for-low-frequencies/ which I thought was quite good.