Interdigital Band Pass filter

[Martyn Kinder]

Hi all

I am building a 5 element 1420MHz Interdigital Filter using the calculator found here:

https://www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php

This provides a theoretical design demanding quite exacting accuracy for the construction. Probably too exacting for my engineering capabilities.

It's been a long time since I last built an interdigital filter. Ideally, I would like to make it tuneable by using tuning screws opposite each of the resonator rods, but I cannot remember whether I need to make the resonator slightly longer or shorter and by how much. Can someone remind me please?

Thanks

Martyn

DESIGN DATA FOR YOUR BANDPASS

Interdigital Bandpass Filter, based on work of Jerry Hinshaw,
Shahrokh Monemzadeh (1985) and Dale Heatherington (1996).
www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php
Javascript Version : 09. Jan 2014
-------------------------------------------------------------------------
Design data for a 5 section interdigital bandpass filter.
Center Frequency          :    1420 MHz
Passband Ripple           :    0 dB
System Impedance          :    50 Ohm
Cutoff Frequency          :    1415 MHz and 1425 MHz
Bandwidth (3dB)           :    10 MHz
Fractional Bandwidth      :    0.0070
Filter Q                  :    142
Estimated Qu              :    1961.04
Loss, based on this Qu    :    2.034 dB
Passband Delay            :    103.007 ns
-------------------------------------------------------------------------
Quarter Wavelength            :    52.78 mm or 2.078 inch
Length interior Element       :    47.54 mm or 1.872 inch
Length of end Element         :    47.6 mm or 1.874 inch
Ground plane space            :    19 mm or 0.748 inch
Rod Diameter                  :    6 mm or 0.236 inch
End plate to center of Rod    :    15 mm or 0.591 inch
Tap to shorted End            :    2.47 mm or 0.097 inch
Impedance end Rod             :    82.756 Ohm
Impedance inner Rod           :    83.597 Ohm
Impedance ext. line           :    50.000 Ohm
-------------------------------------------------------------------------
                     **** Dimensions, mm (inch) ****
 #    End to Center      Center-Center        G[k]         Q/Coup
 0    0.00 (0.000)
 1    15.00 (0.591)      33.50 (1.319)        0.618        1.000
 2    48.50 (1.909)      37.08 (1.460)        1.618        0.556
 3    85.58 (3.369)      37.08 (1.460)        2.000        0.556
 4    122.66 (4.829)     33.50 (1.319)        1.618        1.000
 5    156.15 (6.148)     0.00 (0.000)         0.618        0.618
 6    171.15 (6.738)      

                      **** Box inside dimensions ****
Height : 52.78 mm or 2.078 inch
Length : 171.15 mm or 6.738 inch
Depth  : 19.00 mm or 0.748 inch

---

[Marcus D Leech]

I’ll point out that Bert Modderman makes some excellent filters. 

He’s done custom work for us that is too drawer. 

Sent from my iPhone

On Jun 14, 2024, at 5:42 PM, Martyn Kinder <mar...@czd.org.uk> wrote:

Hi all

--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/8d40dc65-4526-4d0f-9be7-84062fe42e12n%40googlegroups.com.

[Don Latham]

This is nuts? I went to the site, but find only pictures. Where is the actual designer calculator?

What did I miss?

---

From: "marcus leech" <patchv...@gmail.com>
To: "sara" <sara...@googlegroups.com>
Sent: Friday, June 14, 2024 4:30:07 PM
Subject: Re: [SARA] Interdigital Band Pass filter

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/4EB990D2-7DAC-406D-8646-40B24EF59E9D%40gmail.com.

--

------------
Don Latham
PO Box 404,
Frenchtown, MT, 59846
406-626-4304

[Don Latham]

oops. never mind. it was there all the time.

---

From: "Don Latham" <d...@montana.com>
To: "sara" <sara...@googlegroups.com>
Sent: Friday, June 14, 2024 4:52:55 PM

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/1647884619.19921664.1718405575481.JavaMail.zimbra%40blackfoot.net.

[Marcus D Leech]

Sent from my iPhone

On Jun 14, 2024, at 6:55 PM, Don Latham <d...@montana.com> wrote:



To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/2040156361.19925237.1718405749380.JavaMail.zimbra%40blackfoot.net.

[Don Latham]

wow...

---

From: "marcus leech" <patchv...@gmail.com>
To: "sara" <sara...@googlegroups.com>

Sent: Friday, June 14, 2024 5:21:22 PM

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/CF2B01B1-08BF-4D95-A66F-44B7A4A215EC%40gmail.com.

[Marcus D. Leech]

On 14/06/2024 20:43, Don Latham wrote:

wow...

Yeah.  I just thought I'd point out Bert's work, because if one doesn't have all the right tools to make a precision
  inter-digital filter, Bert does excellent work.

He built two custom filters for us -- super low loss (under 0.1dB at band center!) fairly wide-band (200MHz wide
  at the 3dB points).  We mostly wanted to keep the nearby cell site "goop" out of our front-end, to keep things
  nice and quiet and linear...

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/1938173874.20115309.1718412212832.JavaMail.zimbra%40blackfoot.net.

[Mike Otte]

Hey Martyn,

I was just building bandpass filters ,  4 digit comb.

I was using some of the dimensions of S53MV..

Check out   https://s53mv.s56g.net/cavity/cavity.html

I don't have easy access to 40 x60 mm aluminum tube so I am trying  1.5 x 3" alum tube and it seems to be working. I am still tuning. One came up right at 1420 and the other is about 1410 Mhz..  I am learning how to run a mini VNC at the same time.  I want them to be the same to use on my interferometer and I don't want them to be exactly on 1420.406.

these slugs are 8 mm x 44 mm long

BR

Mike

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/1bae0085-2b87-4dde-9c2e-a4edce673945%40gmail.com.

--

Mike Otte W9YS

[James Abshier]

Hi Martyn,

I built a 3-element interdigital filter using the design tool that you referenced. It appears that the length given for the elements are for use with adjustment screws. Although the tool reports values to a lot of decimal places, I don't think that the elements need to be fabricated with that accuracy. Perhaps the closest millimeter might be good enough since you will be tuning it. If adjustments to the element lengths are needed, shortening the length raises frequency and lengthening it decreases frequency.

For my filter, I used 1 inch by 1/4 inch hardware aluminum stock for the sides. This can be drilled and tapped to fasten the sides, top and bottom together. The finished filter is shown in an attached image of a superhetrodyne receiver that I built. I suggest that the screws for fastening the top and bottom plates on be in line with the filter elements so that good contacts are made at high current points. The elements of this filter are brass rod obtained from a hobby shop. Although I used aluminum top and bottom covers, you could probably find some sheet brass at a hobby shop. Another source of sheet brass could be a door kick plate made of brass. I have found these at hardware stores.

A plot of the filter response is also attached. It is better than the popular SAW filter in the NooElec Sawbird. It is nice that people have coded the procedure for designing these filters because it used to be rather tedious.

Good Luck,

Jim Abshier

[Don Latham]

Looking good. I think I may try using prethreaded aluminum standoffs cut to the appropriate lengths.

The calculator is a real joy to use, sit and play until you like it...

Don

From: "James Abshier" <abs...@ameritech.net>
To: "sara" <sara...@googlegroups.com>
Sent: Saturday, June 15, 2024 3:07:53 PM
Subject: Re: [SARA] Interdigital Band Pass filter

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/bb4e736c-5819-4abd-aeca-b1f33fa05112%40ameritech.net.

[Martyn Kinder]

Hi all

Apologies for the delay responding. Just got back from a short holiday and still catching up.  I'll build as per the calculated design, using 6mm tuning rods with M5 tuning screws. If I get it wrong its no big deal to alter the tuning rod length.