[Amps] Advice: harmonics from Alpha 91B on 160

[Robert Groh]

Oh, boy, all kinds of questions to ask here. 

First off - how is he detecting these harmonics?  It would be very easy to overload the detecting device and have the harmonic being produced inside the detecting device (e.g. receiver). Trying to measure harmonic content in close proximity to the transmitter/PA is difficult in a non-lab environment. Using a buddy a couple of miles away would be quite acceptable (if you know the relative antenna gains, etc). 

Second - every practical amplifier will have harmonic content. My mind is rusty on what the current FCC requirements are but I believe it is somewhere south of 60 dB down from the output.  If he is putting out 1 KW means the harmonic(s) could be as high as 1 mW (i.e. +30 dBw out - 60 dB = -30 dBw = 0 dBm) and be entirely within specification.  1 mW is a lot of power!

Third - his antenna will undoubtedly have some (probably substantial) rejection to the 3rd harmonic. So the 'radiated' 3rd harmonic will be knocked down even more. 

Fourth - if he is actually causing interference on the 3rd harmonic and it is not interference caused by close proximity then I would simply recommend he consider a simple low pass filter in his feed line (e.g. with a 3 MHz cutoff). Should be fairly easy to build and would give > 15 dB attenuation.

Hope this all helps in a general sense.

73
Bob Groh, WA2CKY




________________________________
From: gds <gerys1 at verizon.net>
To: amps at contesting.com
Sent: Sat, January 30, 2010 9:51:19 PM
Subject: [Amps] Advice:  harmonics from Alpha 91B on 160


A friend of mine is using an Alpha 91B on 160 and is noticing harmonics  around 5.595 MHz. (3rd order)

Is this a problem that has been noticed with the 91B?  Anyone know of a solution?

Tks very much.

Gerry
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