[Elecraft] Intermod filter for the KX3

[Bob McGraw K4TAX]

If the interference is a product of 2 or more out of band signals mixing 
together, then a band pass filter would help.    If this the case, use 
of noise reduction or noise blanking in the KX3 will likely make the 
issue worse, not better.     One test is to use a step attenuator 
between the antenna and the receiver input.  Thus selecting some value 
of attenuation for all signals, thus equally reducing the 2 or more out 
of band signals.  If this improves your issues with regard to the 
interference, one can attribute to strong signals mixing and entering 
the receiver.  A band pass filter will likely help and allow you to 
transmit on that band as well as receive on that band.   A band pass 
filter will be required for each band you intend to operate.   A full 
set, they aren't cheap.

Likewise on 2M, many paging and public service systems are now 
transmitting digital data streams.  These cause me havoc on 2M as I am 
600 ft from the local 911 communications tower.  Only 12 total of 
antennas, VHF and UHF,  on that tower.   A band reject or notch filter 
corrects the issue.   But again, one needs to know the exact frequencies 
of the signals which are causing the interference as that is the 
frequency the notch must be applied. I use a filter from PAR Electronics 
which is a notch filter.  The insertion loss on 2M is very low allowing 
me to receive and transmit with the filter in the line between the radio 
and the antenna but provides about a 40 dB notch at the offending 
frequency.     In general handheld radios do not have adequate space 
internally to design in and implement good front end filters.   They in 
general are very subject to interference and do not perform well with RF 
from nearby services operating above or below the ham bands.

If the interference is in the pass band of the receiver, finding and 
eliminating the source is the optimum approach.  If the mixing 
components are in the PA of a transmitter and thus being radiated and 
picked up by your receiver, finding the source is about the only 
solution.   This could be the product of 2 FM stations in the building 
or a building of close proximity.   As an example, 99.5 MHz and 106.7 
MHz mixed together produce two signals being a sum and a difference.  
The difference is 7.2 MHz.....smack in the 40M band.   Probably due to 
harmonics affect 20M and 15M to some extent.   Depending on exactly 
where the mixing is taking place, a corroded pipe on or in the building, 
in the PA of one of the transmitters, or some other device which becomes 
non linear when excited with RF, then it radiates the signal.  Finding 
and correcting the issue is about the only solution.

Third choice.............move.     Which in fact might be easier.   Just 
be sure one doesn't move into an RF plagued area.

73

Bob, K4TAX

On Jun 3, 2018, at 12:49 PM, Mike Parkes <mspmail2 at gmail.com> wrote:
>> Hey group
>> I have an issue here and am wondering if a good band pass filter would
>> help? I pick up so much sqawks and buzzing weird interference, grating
>> machine sounds, on my KX3 on all HF bands, it is horrible in my setting
>> (ground floor apartment in the city). I have the same issues on 2 meter
>> handhelds, noise all over the band, trips the squelch making it nigh
>> impossible to scan without the radio stopping on some ghost signal that is
>> only intermod. Same story with the KX3 2 mtr module though I can turn the
>> squelch up enough to keep the radio scanning without stopping on noise for
>> the most part. Is there any hope for HF? Would some sort of HF band pass
>> filter help my KX3? Or is the filtering in the KX3 already good enough? I
>> can't make it go away with the built in KX3 noise filters. There is this
>> one company across the street from me, some internet megalith place with
>> sat dishes on the roof and I wonder if some of the noise is coming from
>> that establishment. Anyway... thanks for any ideas... head for a park, a
>> mountain peak... I know... :)
>>
>> 73's Mike AB7RU DN17gp
>>