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 VHF band pass filters 
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Post VHF band pass filters
Hi guys, I have a question I hope you can help me with my interference problems, I live very close to my local hospital/ emergency room, we have the x-Ray building about 75 meters away, there is a mobile MRI unit parked in the car park and pagers the list goes on and on, it's not as bad as it sounds but on 2 meters MS it's unusable and on FM there is intermittent noise that hits S9, Ofcom said it was an interesting problem.... Read between the lines there. So there must be some thing I can do for myself, I'll spend money if I have to but I'd rather make some thing, but it has to be future proof and be able to handle 100 watts + as and when I pass the full licence, a friend has a DCI filter that helped but I'm not paying big money.

http://www.dci.ca/?Section=Products&SubSection=Amateur

I have a FM hand held that is 100% unusable on the co-linear on the roof, I didn't expect much with the hand held as the receiver is wide open but it would have it work as the XYL payed for it, she keeps asking why I don't use it.


Thu Feb 01, 2018 7:23 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
The hand held, on an external antenna, is a big ask as most of them have terrible receivers.

There are surplus filters around, from the mid band systems. Usually made by Aerial Facilities.
Obviously they need retuning, or modifying, and the right gear to set them up.
They can be a bit of a fiddle.
Ideally they want to be in the RX path only as, to get really good rejection at 140MHz and 150MHz, you'll probably get a couple of dB loss at 144.3MHz.

What equipment are you using for MS?

Cheers

Tony

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Fri Feb 02, 2018 12:16 am
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 VHF band pass filters 
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Post Re: VHF band pass filters
Hi Tony, thanks for the reply, for MS I'm using a Flex 1500 as a 28mhz IF going into a ME2T Proii transverter and 1 2m12, there is obviously no pre-amp fitted although i have one, maybe the 2M12 is to much antenna for where i live but again it has to be, I need to reduce out of band signals.

http://www.ha1ya.hu/htmkepek/me2t_protr ... erspec.htm

http://www.m2inc.com/FG2M12

http://www.eham.net/reviews/detail/8986

On FM I'm using an ID5100 this is not to bad on 2meters but on 70cm it really struggles, the handy lives in my office in the underground car park at work, but that's a thread for another day.


Fri Feb 02, 2018 12:11 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
I wonder if any one has any thoughts about these filters from Antennas-Amplifiers, I don't mind spending the money, but i quite like the idea of getting some thing to work for me.

http://www.antennas-amplifiers.com/Band ... ass-Filter


Fri Feb 02, 2018 12:50 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
Nick wrote:
I wonder if any one has any thoughts about these filters from Antennas-Amplifiers, I don't mind spending the money, but i quite like the idea of getting some thing to work for me.

http://www.antennas-amplifiers.com/Band ... ass-Filter

That looks a bit flat to me.
The main problems can be signals in the 138 - 140MHZ, and 155 - 170MHz areas.
So you need to make sure there's appreciable attenuation that close in.

Cheers

Tony

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Fri Feb 02, 2018 1:35 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
I agree with Tony that the passband of the one you listed is too wide to be of much use to you.

I had one from Ocicon in Canada years ago for use in contesting as I had a few out of band issues from a local high power system at one spot I used, It was tweeked for the UK band plan rather than for stateside 144 - 148Mhz and it's response was even was sharper than the one they still advertise;
http://www.ocicom.com/index.php/amateur ... ass-filter
I've still got it up in the storage but keeping for "just in case" :D

Similarly if you feel like putting one together yourself it's not hard at all and there are a good few designs out there that give quite tight performance
e.g. http://yu1lm.qrpradio.com/2m%20BP%20FILTER-YU1LM.pdf

Similarly if you are using split i.f. on your transverter system like me you can just use the filter on receive if that is where your issue is and downrate the components as they'll have no power requirements to speak off.

73
Kelvin

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Fri Feb 02, 2018 1:58 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
Thanks for the replies,they are very interesting, aparantly the guy who designed the Ocicom filter designed the DCI filter, the Ocicom filter can take 160 watts into it so can after the amplifier,as far as a split IF is concerned it's an idea for another day, I need to find a answer to the fundamental problem.



http://www.ocicom.com/images/OCI/PROD-REV-OCI.PDF


Fri Feb 02, 2018 8:09 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
Nick wrote:
Thanks for the replies,they are very interesting, aparantly the guy who designed the Ocicom filter designed the DCI filter, the Ocicom filter can take 160 watts into it so can after the amplifier,as far as a split IF is concerned it's an idea for another day, I need to find a answer to the fundamental problem.
http://www.ocicom.com/images/OCI/PROD-REV-OCI.PDF


Relatively new to HRD hence late reply.

If you can put a filter in the Rx path, but not Tx through it, it might be worth considering a notch filter to remove the qrm.
They can be very sharp. It depends if the qrm is quite narrow.

I have pager qrm with my weather sat set-up and built a couple of notch filters.
The design I copied is http://www.satsignal.eu/wxsat/filters/HelicalNotch.htm
My construction at http://www.g4iat.co.uk/wx/meteor/notch.html
I built 2 of these notch filters and connected them in series.
-40dB at the unwanted frequency, about -2.3dB 0.9MHz away from the notch.


Wed Apr 11, 2018 3:18 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
What about 1/4 wave stubs?

Cheap and can give up to 50dB attenuation.

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Wed Apr 11, 2018 3:56 pm
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 VHF band pass filters 
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Post Re: VHF band pass filters
Sangoma wrote:
What about 1/4 wave stubs?

Cheap and can give up to 50dB attenuation.

That's the simple, and cheap, solution. If it's spot frequencies that need to be removed.
Just trim for minimum. :)
Often used in conjunction with high pass, and low pass, filters in duplexers.

Cheers

Tony

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Thu Apr 12, 2018 8:43 am
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 VHF band pass filters 

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Post Re: VHF band pass filters
Aren't quarter wave coaxial stub notches more like 30-35dB and also quite wide?
I'm currently looking for the same solution as I get pager QRM on my site on 2m from a 139MHz pager. I got a vhf designs bandpass before the source was identified. It made no difference as the source was so close to the passband.
Can anyone plot a coaxial stub? Unfortunately I have moved jobs from a company with VNAs and all I needed to a company where a DVM is rocket science. On the upside it's a metal bashing company which is EXTREMELY useful for my homebrew stuff!

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Mon Aug 12, 2019 6:47 pm
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