[RFI] For Hams too close to a AM station

[AA5CT]

Thank you Jim.

It is designed for 100 Watts above roughly 1830 kHz on account of
high circulating currents in components below that point.The idea was
that some radio installations need HP filters on the transmitter output
to avoid intermodulation with nearby AM band transmitters back
into a ham band radio PA stage. I've used this filter for that purpose,
as I have a 5 kW AM station at 1700 kHz several miles to my ENE
and noticed 'mixing' when my WSPR xmtr came on the air when
working 160 meters.

All measured performance values were in a 50 Ohm system using
HP lab gear. I can't vouch for performance going above or below 
that value, or for highly reactive 'loads' as some receivers may present.

de AA5CT Jim

-----------------------------------------



On Tuesday, October 26, 2021, 5:04:30 PM CDT, Jim Brown <jim at audiosystemsgroup.com> wrote: 





VERY nice looking filter, Jim, and as good or better than any commercial 
product I've seen advertised. Is your build designed to handle TX power, 
or RX only?

One caveat -- filter performance depends on both source and load 
impedance, which are controlled in the VNA, but are not in some ham RX 
front ends, and may not be in the source. I've measured input Z of my K3 
RX, and learned that they are 50 ohms with a bandpass filter in front of 
them for each band.

73, Jim K9YC

On 10/26/2021 2:49 PM, AA5CT via RFI wrote:
> 
> Or, build this model and save a few bucks! This high pass model
> yields 40 dB attenuation starting at 1700 kHz and gets better further
> down in the band.
> 
> Article comparing performance with existing filters (at the time):
> 
>  https://www.qsl.net/wb5wpa/Filter/AM Brick-Wall 160 Meter Filter.pdf
> 
> Schematic and performance:
> 
> https://www.qsl.net/wb5wpa/Filter/DesignAMBrickWall_08.pdf
> 
> 
> BTW, the RigExpert AA-55 Zoom will perform within 1 kHz of a strong
> AM carrier (like, measured 0 dBm or 1 mW on the antenna) from say a
> station at 1700 kHz and tuning the antenna for same. If the AA-55 is tuned
> to 1700 kHz exactly one sees a 'beat note' (bouncing RL value) on the meter.
> They do not use a broadband detector in that analyzer.

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