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eHam Forums => Amplifiers => Topic started by: NO9E on February 21, 2013, 11:20:32 AM



Title: Second harmonic when operating 160m
Post by: NO9E 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


Title: RE: Second harmonic when operating 160m
Post by: AA4PB 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.


Title: RE: Second harmonic when operating 160m
Post by: N3QE 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.


Title: RE: Second harmonic when operating 160m
Post by: KE3WD 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


Title: RE: Second harmonic when operating 160m
Post by: AD4U 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


Title: RE: Second harmonic when operating 160m
Post by: N8CBX 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


Title: RE: Second harmonic when operating 160m
Post by: W4OP 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


Title: RE: Second harmonic when operating 160m
Post by: G3RZP 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.


Title: RE: Second harmonic when operating 160m
Post by: NO9E 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.   


Title: RE: Second harmonic when operating 160m
Post by: N4OGW 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


Title: RE: Second harmonic when operating 160m
Post by: N3QE 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.


Title: RE: Second harmonic when operating 160m
Post by: NO9E 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


Title: RE: Second harmonic when operating 160m
Post by: G3RZP on February 22, 2013, 08:08:18 AM
But it would be an awkward Z on 10 MHz.


Title: RE: Second harmonic when operating 160m
Post by: W4OP 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


Title: RE: Second harmonic when operating 160m
Post by: N4OGW 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


Title: RE: Second harmonic when operating 160m
Post by: G3RZP on February 23, 2013, 05:45:27 AM
I have an 80m dipole fed with open wire line 60 feet long, so it's high Z on 80 and 40. On 30, it is surprisingly low Z, judging by the feeder current. Old fashioned link coupled tuners, located in a shed at the base of the tower and the tuners motor driven. Much easier!


Title: RE: Second harmonic when operating 160m
Post by: W4VR on February 23, 2013, 08:33:34 AM
Could also be a bad connection at the antenna.  I heard someone on an 80 the other day who was operating a CW contest on 160.  I sent him an email and he found the problem...a bad connection feeding his inverted L.


Title: RE: Second harmonic when operating 160m
Post by: KC4MOP on February 23, 2013, 04:32:45 PM
I kinda go with the some of the other observations. 2nd harmonic and you would see very high SWR on your feed to the antenna.
I accidentally mis-tuned a transmitter on 160M and it turned out to transmit something on 80M. There was power and very high SWR. The 160M freq and the exact doubling to 80M.


Title: RE: Second harmonic when operating 160m
Post by: G3RZP on February 24, 2013, 03:24:36 AM
KC4MOP,

If the second harmonic was 40dB down and the SWR at the second harmonic was infinite, the swr you would see caused by harmonic power would be 1.02:1


Title: RE: Second harmonic when operating 160m
Post by: KL7CW on February 27, 2013, 12:41:53 PM
Ignacy,
    You did not say how you made your measurements.  I would suggest you make measurements into a dummy load with an appropriate sample device.  Verify that your 2nd harmonic is down more than 43 db from the carrier.  If not check the transceiver, although the output filter in the amp should "in theory" clean up much of the second harmonic energy going to the dummy load on the amp....however it is generated. 
      If your inverted L is resonate on 160 meters with something like an SWR of less than 2:1  (< 1.5:1 is better), then you may be able hook it directly to the amp(through coax ?), however it is unlikely that this is the case with your 150 ft inverted L and radial system, unless you happen to be very lucky. If the antenna is tuned to resonance with an L network, then it will present a 50 ohm non reactive load to the amp on 160 meters. 
      Now your antenna will radiate power "efficiently" on 160 meters, but be a rather poor radiator on 80 meters, possibly 10, 20, or more db down in efficiency.  Thus an amp with -43 dbc 2nd harmonic output, with the above antenna, might only radiate something like -50, -60, or -70 dbc on its 2nd harmonic. However, it is still possible for say -50 or -60 dbc radiated signal to be heard at a distance of 80 miles. 
       If you use an antenna tuner, read up on L and T network tuners.  I believe an L network tuner will do a much better job of suppressing 2nd harmonic energy.  A stub or an LC trap designed to suppress 80 meter signals will work, however most tend to be rather sharp with good suppression only very near their design frequency. 
       I doubt that the fault is typically in the receiver.  Typically the worse receiver images are at frequencies other than 0.5 X the rx frequency.  This assumes that the receiver is not in the immediate vicinity of the transmitter....like in a field day set up. 
      So if your antenna, and amp are OK as suggested above, go ahead and enjoy 160 meters and remember 0.15 watts or even 0.015 watts radiated on 80 meters can be easily heard on 80 meters.  On 80 meters, a significant part of your radiation from your inverted L will be from the horizontal section, which makes a nice NVIS radiator.  QRP NVIS signals on 80 meters tend to often be very strong out to perhaps 200 miles, or sometimes further. 
          Rick   KL7CW     Palmer, Alaska        Ham for 59 years    many years in electronics industry