Second harmonic when operating 160m

[G3RZP]

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!

[W4VR]

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.

[KC4MOP]

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.

[G3RZP]

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

[KL7CW]

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