Bandpass Filter

[bcrosby]

Hello,

Did a search on the forum and Dave made a suggestion for the ABF125 filter that you can purchase at universal radio (http://www.universal-radio.com/catalog/filters/0880.html).

Two questions.

1. Has anyone in Canada purchased anything from universal radio, and if so, how much was shipping and how was your experience?

2. Does anyone know if this filter (or similar) is available from a retailer in Canada? Just trying to avoid duty and shipping costs.

I'm getting quite a bit of interference (from FM Radio) on a tower frequency that I would like to monitor (119.9), thankfully its only the backup freq when the tower is busy. However, for completeness I want to add it to the scanned frequencies.

This is all for a new feed that should be up shortly for Toronto Buttonville (CYKZ)

Blake

[BerendBotje1]

Hi Blake,

Maybe there is a simple solution for filtering out this interference, I'll try to explain in my best English how I solved a similar problem at my place.
I've used a BNC-T splitter in my coax line from the receiver to antenne in combination with a tuned piece of coax cable. See the picture below, don't know the English name of this "filter"



At the T of the splitter use a tuned piece of coax cable at the frequency you want to filter. The lenght is calculated as follows. (all lengths are metric metres)
For example the radiostations frequency is 105.1 Mhz and the coax on the T split is a RG 58 type with a Vk of 0.66. (See this table voor specs specifications)
Take 300/105.1 = 2.85 (wavelenght)
Cable length is then (2,85/4) x 0.66 = 0.47 metre.

Start with a length, measured from the centre of the T split, of 0,50 metres ( 50 centimeters).
Tune the receiver on the 105.1 and cut tiny bits on the end of the coax with a sharp knife untill the signal is at it's most weakest. Don't connect the core and the shield of the coax, make sure the line is "open"
All other signals are now received mostly normal, only the tuned one is filterd out.

I don't know if this is the sollution for your problem but I gues it's worth  a try since the costs are very low. 

Grtz David

Edit: added pics below.


BNC T split with tuned coax cable


Used in combination with a SP-3 combiner/splitter.

[bcrosby]

Hi!

Thanks for the details. The problem is that yours tunes out a specific frequency. As with what I want to do is filter out EVERYTHING except 108Mhz-118Mhz.

These instructions might be helpful for someone else though

[BerendBotje1]

Yep that's true..missed that one..

Good luck anyway with the setup!!

[SkyViking]

Blake, regarding the bandpass filter, have you tried the hugh radio/scanner/ham-operator store on Steels Ave.&400?  They seem to have everything under the sun related to this hobby.

Cyrus

[dave]

Hi!

Thanks for the details. The problem is that yours tunes out a specific frequency. As with what I want to do is filter out EVERYTHING except 108Mhz-118Mhz.

These instructions might be helpful for someone else though

Blake-

Here is another source for bandpass filters - just stumbled on these:

http://www.scannermaster.com/BPF_AIR_Air_Band_Band_Pass_Filter_p/24-531022.htm

http://www.scannermaster.com/Filters_s/39.htm
Note: They have a couple of notch filters as well, one for the FM radio broadcast band and one that is tunable.  Could be very handy for certain listeners who are close to high power transmitters.

Their filters are around $30 USD each - which isn't bad at all.  Especially with the lousy performance of the U.S. Dollar these days. 

They are a local Boston company...I may pay them a visit soon to see if we can get any package deals for LiveATC feeders.

Dave

[dave]

Hi Blake,

Maybe there is a simple solution for filtering out this interference, I'll try to explain in my best English how I solved a similar problem at my place.
I've used a BNC-T splitter in my coax line from the receiver to antenne in combination with a tuned piece of coax cable. See the picture below, don't know the English name of this "filter"
...
Used in combination with a SP-3 combiner/splitter.

The filter you describe is an open quarter-wavelength stub filter.  A stub is a length of transmission line terminated in a fixed impedance, usually via a short-circuit or an open-circuit, used in parallel with another transmission line to create a unique tuning or canceling effect.  These unique effects work with frequencies that are harmonically related (multiples of one another).

Practically, in your example, this filter is great for nulling (canceling) the FM station at 105 MHz, but it will create a filter that is designed to pass signals around 210 MHz (2 x 105 MHz).  It may do an adequate job of still passing signals at airband, but I would be interested to know what your experiences have been using this filter compared to no filter at all.  How much does it attenuate airband frequencies?   In any case, you are correct that it is a cost-effective filter, and fun to play with if any of the readers are so inclined.

I'll try to post more on this topic soon, but for those with this problem I seriously recommend this filter:

http://www.scannermaster.com/HPN_30118_Combined_Notch_Filter_p/24-531024.htm

This filter will greatly reduce or eliminate interference from both AM and FM radio stations, TV stations, and shortwave stations.  It's not just designed for airband - it will pass all VHF and UHF frequencies...so great for general scanner monitoring.


Dave

[BerendBotje1]

The filter you describe is an open quarter-wavelength stub filter. 
............
It may do an adequate job of still passing signals at airband, but I would be interested to know what your experiences have been using this filter compared to no filter at all.  How much does it attenuate airband frequencies?   In any case, you are correct that it is a cost-effective filter, and fun to play with if any of the readers are so inclined.

Yep, that's the one, open quarter-wavelength stub filter. 

I've used the stub filter at a aeroclub on a airport with two scanners, Tower(118.700)/Approach(120.300) and Ground (121.700). A continues METEO broadcast (126.200) about 500 metres away interfered with the reception of aircraft further away on the tower frequency and now and then even on the ground frequency. In my opinion the interference was caused by the less selectivity of the scanner (Uniden Bearcatt 9000xlt). After applying the stub filter the interference of the METEO broadcast was gone. The other trafic in the CTR is received a little bit less then without the stub, but the conversations are better heard and understood.

Audioclip without filter
Audioclip with filter

It might not be the solution for all the problems but in upper case it worked out fine.

Grtz David

[bcrosby]

Blake, regarding the bandpass filter, have you tried the hugh radio/scanner/ham-operator store on Steels Ave.&400?  They seem to have everything under the sun related to this hobby.

Cyrus

Cyrus,

No I haven't. I should drop by as its only a 20 min drive from my place. Thanks!

[bcrosby]

I'll try to post more on this topic soon, but for those with this problem I seriously recommend this filter:

http://www.scannermaster.com/HPN_30118_Combined_Notch_Filter_p/24-531024.htm

This filter will greatly reduce or eliminate interference from both AM and FM radio stations, TV stations, and shortwave stations.  It's not just designed for airband - it will pass all VHF and UHF frequencies...so great for general scanner monitoring.


Dave

Dave,

Interesting.. this sounds like the winner to me! $30 is cheap.. unfortunately they charge $19 (66% of the cost of the filter) for shipping to Toronto.

However, I'm going to be in Boston in early March. I might as well just pick a few up while I'm down there

[KB6HLM]

Ya thats a bargain compared to the $90 bucks I shelled out for mine ! at http://www.stridsberg.com/prod02.htm

Although it was worth it to me do to the fact I have a preamp inline with 4 Radios connected (expensive ones) to it and I like the specs 
Notch:  > 60 dB @ 98 MHz
Insertion Loss:  < 1 dB @ 500 MHz

However if don't have thousands of dollars worth of equipment I would recommend getting the least expensive one with the best specs and $30 bucks for insertion loss: 1 dB typ suppression in the range: 0 - 20 MHz: 30 dB min
suppression in the range: 87 - 109 MHz: 30 dB min

NOT BAD at all !!

73s
Glenn KB6HLM Las Vegas, NV