G8JNJ

Hi Lonecrow,

If they are fairly short patch cables it won't make much practical difference. Ideally you would use the correct impedance throughout, but it's less critical for receive only purposes.

Note that frequencies on the airband commonly have off-sets to accommodate 8.66KHz channel spacing within 5KHz frequency steps supported by older aircraft radios and also to allow multicasting from several sites on the same frequency (typically for wide area stuff like Volmet).

Aircraft and some ground based transmissions (typically at smaller or grass strip airfields) also have slight frequency errors that can often be observed on spot frequencies.

Regards,

Martin - G8JNJ

Hi Lonecrow,

Yes you can use the airband and FM band filters together, if the airband filter is good enough you may not need the FM band notch.

I think the reason you don't see it with the dongle and SDR Sharp may be an issue with the KiWi that you probably wouldn't otherwise notice, but this is what I'd hope to test.

To get the frequency off-set correct you may need to either adjust the transvertor reference frequency and / or the KiWi Frequency scale offset (kHz) value.

Regards,

Martin - G8JNJ

Hi Lonecrow,

I think the problem could be a very strong out of band VHF Digital TV signal that is being down-converted to a frequency that is being aliased by the KiWi to the frequency range you are seeing.

The reason for trying the Airband filter is to see if it reduces the unwanted signal that may not be suppressed sufficiently by the transvertor's built in filters.

The 30MHz low pass filter will help test to see if the problem is the down-converted un-signal that is being aliased by the KiWi sampling frequency from somewhere around 70MHz to 18MHz.

As I say, it is only a theory at the moment.

Regards,

Martin - G8JNJ

Hi Lonecrow,

Looking at the spectrum and using the new peak hold function, the noise has a fairly well defined 'shape' that could possibly be a VHF Digital TV transmission using the ATSC standard.

I have a theory about how this could be occurring, but a bit of testing is required to prove it.

First - does the noise go away when you disconnect the Air Band antenna ?

If it does then try.

1. Adding an aircraft band Band Pass Filter to the input of the transvertor.

Something like the "bandpass filter BPF 118-136MHz for Air band" that are sold on Ebay.


2. Adding a 30MHz low pass filter between the transvertor and the KiWi.

Something like the "ZETAGI F27 FILTER ANTI TVI FREQUENCY 0-30 MHZ 300W" or some other CB TVI filter as sold on Ebay


Do either (or both) of these things appreciably reduce the level of the interference ?

Regards,

Martin - G8JNJ

Hi John,

Thanks for fixing it.

I only accidentally noticed it when I realised that the auto notch was no longer removing a strong carrier.

Regards,

Martin - G8JNJ