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Author Topic: Second harmonic when operating 160m  (Read 3322 times)
NO9E
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Posts: 403




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« on: February 21, 2013, 11:20:32 AM »

When operating in CQ 160 CW, I was told that my signal was heard  on 80m some 80 miles away. In fact, that person heard many 160m signals on 80m.

My amp is TT Centurion. I made measurements and it seemed that harmonic suppression was a bit better than -40db. This is worse than -50db in QST review.

I suspected a toroid saturation in my inv L but the level of suppression was similar at a lower power level. Could extra harmonics be generated by ceramic padding capacitors?

Not sure the problem is big to be bothered. One solution would be to add an L network for inv L. However, this inv L does pretty well on 30m!

Ignacy, NO9E
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AA4PB
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« Reply #1 on: February 21, 2013, 11:24:21 AM »

If that person hear **many** 160M signals on 80M then maybe the problem is on his end.

I'd get someone else to run some tests with you to confirm that it really is you that has the problem before starting to make any changes.
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N3QE
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« Reply #2 on: February 21, 2013, 11:30:20 AM »

I agree, if he is hearing "many" 160M signals on 80M, then the problem is on his end.

Many receviers have weak front ends to begin with, and they can be made even worse if the ham goes in and disables the BCB-reject filtering that was supposed to be in the RX.
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KE3WD
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« Reply #3 on: February 21, 2013, 11:50:09 AM »

Ye Olde "Everybody on the band is wrong but me" syndrome. 

Ignore the noise, look for the signal.


73
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AD4U
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« Reply #4 on: February 21, 2013, 12:14:33 PM »

I would not be overly concerned.  Maybe "he" has a problem on "his" end.  Checking your rig with a spectrum analyzer will tell you what need to know.

If you are running 1500 watts and assuming your second harmonic is "down" 40 dB, you will still be transmitting a 0.15 watt second harmonic.  If your second harmonic is "down" 30dB then your second harmonic will be 1.5 watts.  Under good conditions 1.5 wats or even 0.15 watt can be heard a very long way.

Dick  AD4U
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N8CBX
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Posts: 147




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« Reply #5 on: February 21, 2013, 12:23:23 PM »

This is nothing new, I have heard 160M key clicks on 80M as well as listening to others saying that they hear them too (at a faint signal level). I actually heard some during the recent 160M contest a week or two ago.
I would think a Pi-L net, tank circuit in a legal limit, 160M amp would clean up any harmonics for good.
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
W4OP
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« Reply #6 on: February 21, 2013, 03:09:59 PM »

I copied perhaps 15-20 160M CW stations on 80 m during the contest. Running a K3 with narrow roofing filters.

Used to hear the same when I had my Icom 7700. I hear the same from my 75M friends. Everyone in the group copies them.
Dale W4OP
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G3RZP
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« Reply #7 on: February 22, 2013, 02:36:06 AM »

Your harmonics must be at least 43dB down to meet Part 97.

When the FCC changed the rules, they said it was to meet new international requirements. Sadly, they got it wrong - the international requirement for power over 5 watts is 50dB down.

What's the toroid for? If it's effectively a broad band transformer, you may well get better results anyway with a tuner.
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NO9E
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« Reply #8 on: February 22, 2013, 04:29:02 AM »

Interesting discussion.

I tried to measure the harmonics with reverse beacons. It seemed that the transmitted power was about 0.1 W. Such a  power with a good antenna can go a long way.

Quote
Your harmonics must be at least 43dB down to meet Part 97.
   

I looked at specs of many amps in QST review and many have <=45 db suppression. Indeed insufficient although any interference caused by the harmonics are likely negligible in this case.

Quote
What's the toroid for? If it's effectively a broad band transformer, you may well get better results anyway with a tuner.

Initially the inverted L (80f vert and 70m horizontal) had only 70 ft elevated radials. The performance was good but all baluns overheated. With a transformer made of FT240-61, there is no sign of heating. I have since expanded to 8 radials.   
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N4OGW
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« Reply #9 on: February 22, 2013, 05:56:02 AM »

Simple solution: add a coax stub to the feedpoint of the inverted L (1/2-wave at 80m, shorted).

Tor
N4OGW
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N3QE
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« Reply #10 on: February 22, 2013, 06:20:07 AM »

I tried to measure the harmonics with reverse beacons.

I have seen reversebeacon spots on 2nd harmonic between different bands before (some 160 to 80, but most often 80 to 40 and 40 to 20). In each case the spotting node was identifying *many* images and harmonics. I think the problem in these cases were not in the transmitters, but in the spotting receiver.

Obviously transmit harmonics can be a problem (and I've gotten OO postcards over the past half century to show it) but I would not get too excited about reversebeacon reports.

My assumption whenever the receiving station is reporting many harmonics from multiple stations... is that the problem is very likely front end overload in the receiver.
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NO9E
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Posts: 403




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« Reply #11 on: February 22, 2013, 06:21:58 AM »

Simple solution: add a coax stub to the feedpoint of the inverted L (1/2-wave at 80m, shorted).

Tor
N4OGW


Tor,
Smart and simple solution. Probably the attenuation will vary by frequency but should > 10db.
Ignacy, NO9E
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G3RZP
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« Reply #12 on: February 22, 2013, 08:08:18 AM »

But it would be an awkward Z on 10 MHz.
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W4OP
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« Reply #13 on: February 22, 2013, 09:19:07 AM »

Usually a 160M antenna is a horrible match for 80M. eg, a half wave dipole on 160M would be a voltage fed antenna on 80M with a huge VSWR. Same for a quarter wave vertical or an inverted L. I can only imagine how loud the harmonics would be if the offending station were running say a 160/80 fan dipole.

Don't think my K3 was overloading......

Dale W4OP
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N4OGW
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Posts: 302




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« Reply #14 on: February 22, 2013, 10:47:13 AM »

But it would be an awkward Z on 10 MHz.

Yes! I have stubs on many of my antennas because I do SO2R during contests. It does unfortunately make it very hard to use antennas on multiple bands.

Tor
N4OGW
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