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

Fri Mar 15 13:42:23 UTC 2019

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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