EMRFD Message Archive 10760

Message Date From Subject
10760 2015-02-06 09:39:49 johnjdignan Help with VHF band pass filter?

HI All,

Can anyone please help me with VHF BPF's?- I've tried the dark art of filter design but the results simply don't match the theory - I lack the proper test gear as well as the knowledge I'm afraid.

If someone could build and test some for me at a reasonable price, I'd love to hear from you.

I need BPF for:

UK 6M band

UK 4M band

UK 2M band

UK 70 cm band

Airband (118 -136)

Broadcast band (88-108)

Ideally be able to handle 1.5 to 2 watts.

I would also like LPF for the above for 25 watts.

Many thanks

John M0JJD

10761 2015-02-06 10:39:55 Ashhar Farhan Re: Help with VHF band pass filter?

i was pretty scared of vhf testing too. but i realized that it was much easier than i thought. 
what you need are two things :

a. power meter that will take you an hour to make. search for the W7ZOI meter.
b. a vhf signal generator. buy any of the excellent Si570 kits.

I have built some band pass filter without any trimmers of gimmick capacitors. wes has documented these. you can read about them at http://w7zoi.net/bpf-ese-nhi.pdf.

I have learnt a simple trick.  

Consider that you want to tune the filter at 2 meters. Connect the power meter to one end and the other end can even be your 2 meter handheld running low power and a 20 db attenuator. Now, follow this carefully:
1. Unsolder the ground end of one inductor
2. hold the unsoldered ground end with a long nose plier and drag on the ground plane looking for the best response on the power meter. solder the coil down wherever it gives the best response. you will need to grow an extra pair of hands for this.
3. repeat for all the coils.
4. you may have to tweak all coils once more to squeeze out the best performance.

- f

10763 2015-02-06 14:44:36 Sandeep Lohia Re: Help with VHF band pass filter?

U down' t actually need silver mica caps for VHF 2M, even easily
available 3-27 PF variable polyester film caps will do the job for
BLY 89A (25 W ) , wound approx 2 turns on regular pencil, & U know
next, Here the amp is 100% home brewed & working from years.

make a 50 ohm dummy load with some 2 watt CFR in parallel & a RF
probe using diode, or follow Farhan.
some data is also there in YU1LM ' s page.

take polymer or wooden pencil like structure, sharp it' s head as
to tune variable caps, avoid metal screwdrivers,
& modern RTL dongle will be a handy tool...

hope final power BJT or FET is SWR protected : D

10766 2015-02-07 02:43:30 g3wie Re: Help with VHF band pass filter?
Hi John,

I may be able to help you get equipped. I have an Si570 generator you could have on extended loan, which would cover everything except 70cm. SMD power meter ics are pretty cheap and I could solder a £12 of those for you to give you a pretty good detector that covers about -10 to -60 dBm with linear

I've used Wes' filter design programs in the 4m transverter I've designed. Happy to show you around those. There's also a neat trick in EMRFD for converting the small value "gimmick" top coupling capacitors into more tractable values; you replace top coupling with bottom coupling which results in larger capacitor values. I have a spreadsheet which calculates it for you. Also, the mixed-form filter which Wes' programs wil design for you,  produce beautifully symetrical filter responses.

I live not a million miles from you in Southampton and will be at the rally this Sunday in Didcot which is close to you. Drop me an email via yahoo if this interests you, perhaps we could meet up on Sunday?


Chris G3WIE
10771 2015-02-09 09:30:41 johnjdignan Re: Help with VHF band pass filter?
HI Chris,
Just got back to my messages today, sorry we missed each other at Didcot! Whilst there, an old friend of mine from Bolton Wireless Club gave me a table of component values for the bands I need, I've made the first (4M) already and it works a treat! So my problem is solved!. Thank you for the very kind offer of your generator, If it has a sweep function (if only up to 2m) it would be help a great deal so I can see what the shape looks like. (i'll worry about 70cm later). If not then no worries. I'm grateful for the gesture nonetheless
Best regards
John M0JJD