[Boatanchors] Eliminating T60 harmonics

[JAMES HANLON]

Y'all,

If you have concerns about the harmonics from any vintage transmitter, a simple way to add about 30 dB of attenuation to the second harmonic output, 48 dB to the third harmonic, 60 dB to the fourth harmonic, etc., is to run its output through a Half-Wave Filter.  They are described in many places, Lew McCoy wrote about them several times in QST, they appear in my Radio Handbook, 15th edition, and you can find them in the November-December 1949 GE Ham News at http://www.steampoweredradio.com/pdf/ge%20ham%20news/1949/nov%20dec%201949.pdf .  If you use the GE Ham News version and are working into a 50 ohm coax line, you will want to build just the top half of what they describe as a balanced, 100 ohm filter.  I use such filters for 80, 40 and 20 with my Collins 32RA8.

One way to tell if you have a harmonic problem is to check your SWR reading.  I use a tuner to set the SWR that my transmitter sees looking into the coax input of the tuner as 1.0 to 1, driving the tuner from a transmitter that I have confidence has essentially no harmonic output.  If I switch to a rig, like my 32RA8 that does have a harmonic problem, the indicated SWR goes up.  That's not because there is a change in the reflected power at the fundamental frequency but because the harmonic power is being reflected.

I do use a Murch 2000 tuner with all of my rigs which may help with the harmonic attenuation, but I have never had a harmonic problem either indicated by SWR or via a Pink Ticket with my Viking Adventurer, Heathkit AT-1, DX20, DX40, DX60 or DX100, Globe Scout 680 or Globe Chief 90, Lettine 240, Gonset Commander, or Globe King 275.  (The Globe King was interesting to neutralize.  Back when channel 2 was on analog tv, I used to do a final adjustment of the neut capacitors to minimize the channel 2 TVI with the Globe King on 20.  It was quite sharp.)

73, Jim W8KGI