[amsat-bb] Shoppinglist (Making a T notch filter)

[Hans BX2ABT]

Hello all,

Unfortunately it's not one transmitter, but four of them (I thought 5, 
but I just checked. Powers range from 1 - 10 kW), producing a lot of 
spurious signals and raising the noise floor by about 10 dB when 
pointing the beam into that direction. I don't think stubs will be broad 
enough to cover 89 to 106 MHz, but I never worked with them, so I'm 
guessing here. I don't have any problems with FM stations leaking onto 
70 cm, just the air band, NOAA frequencies and 2 meters. My Airspy mini 
also picks up lots of spurious signals  below 50 MHz, often hard enough 
to ID.

I found this filter from Par Electronics 
(http://www.parelectronics.com/fm-broadcast.php), but it doesn't 
indicate if you can tx through it. Otherwise I will have to use it in 
the shack and remove it when I want to tx on 2m, with the risk of 
forgetting it and blowing it up. Will a sequencer be a solution then?

73 de Hans

BX2ABT


On 03/15/2019 09:42 PM, Robert Bruninga wrote:
> And you can make it without all the connectors at the T if you want.
> Though it takes some close and inventive soldering of the "T".  Since the
> FM band is 88 to 108 that (I think) is far enough from 145 to add minimum
> loss.  But at UHF, one needs to carefully check to make sure that the FM
> frequency is not also a multiple of the FM frequency.
>
> Tell us the FM broadcast frequency and we can take a quick look.
>
> Bob
>
> -----Original Message-----
> From: AMSAT-BB <amsat-bb-bounces at amsat.org> On Behalf Of Wendy and Terry
> Osborne
> Sent: Thursday, March 14, 2019 11:50 PM
> To: amsat-bb at amsat.org; Hans BX2ABT <hans.bx2abt at msa.hinet.net>
> Subject: Re: [amsat-bb] Shoppinglist
>
> Hi Hans,
>
> If you have a plenty of RG213 and suitable connectors, you could try
> making a Coax stub filter.
> You need a cable length that is an odd multiple of a quarter wave length
> long at the FM Tx frequency and an even multiple of a quarter wave at the
> frequency that you want (2M /70CMs).
> A single RG213 stub would have about 25dB of rejection and would pass 100
> Watts OK.
> You just need a coax T connector and some matching connectors.
> To trim the stub, use a set of garden secateurs.
> If you have a Bao feng or similar radio you could use that on the FM band
> attached to your T and trim the stub for minimum signal on the unwanted Tx
> frequency.
>
> I haven't run the calculations for how long the stub should be but if it's
> useful I could do so.
>
> 73,
> Terry Osborne ZL2BAC
>
>
> -----Original Message-----
> From: Hans BX2ABT
> Sent: Thursday, March 14, 2019 12:33 AM
> To: amsat-bb at amsat.org
> Subject: [amsat-bb] Shoppinglist
>
> My sis-in-law will be visiting from the States soon and I usually make her
> bring some stuff for me so I can save on freight costs. She will
> definitely bring an Arrow Antenna so I can start some portable operations.
> A Sony voice recorder is also on my list, as well as some coax cable
> (RG-400 is what I'm thinking of. Really expensive here).
> Don't have a budget for much more, but since I have a clear line-of-sight
> to some 10 kW FM transmitters from my QTH I thought I should at least get
> a good FM-band notch filter.
>
> Any recommendations on where to buy this in the US? I haven't found
> anything when I googled, not even a lot of 2/70 band pass filters, or low
> pass filters to prevent desensing on 70 cm. My own creations have a too
> high insertion loss, so this time I really want something a bit more
> professional and something that can handle at least 100 Watts of power.
> Any recommendations are welcome. Cheers. --Hans BX2ABT
>
>
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> to all interested persons worldwide without requiring membership. Opinions expressed
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