[NLRS] More on 902 junk

[John P. Toscano]

Chris Elmquist (NØJCF) wrote:
> 
> On Saturday (09/29/2007 at 04:19PM -0400), W0ZQ at aol.com wrote:
>> If you live in the big 
>> city,  if you seem to be a bit deaf on 902, try a good cavity filter.
> 
> Ah.. yes please.  Where do we get them and how soon can we do a group
> buy? :-)
> 
> Things were pretty much OK here until a new digital service recently
> appeared just down band.  It's located on the White Bear water tower.
> Now I can't hear a thing when I look to the S, SW except lots of digital
> modulation all over everywhere.
> 
> Chris NØJCF EN35

Chris:

I wondered the same thing, i.e. where to go shopping for one?

I instinctively went looking on eBay, and the pickings were pretty slim 
indeed for cavity filters.  I did find one that might be up to the task:
 
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=300156368549&ssPageName=STRK:MEWN:IT&ih=020

It seemed to be reasonably inexpensive, and was a Buy-it-Now item (no 
bidding required).  I waited a few days, and no one else snatched it up, 
so I did the only reasonable thing and bought it myself. Once it gets 
here, we'll see how well I can re-tune it to the appropriate ham 
frequencies. I may need a little help, since I have neither spectrum 
analyzer nor sweep generator, but I can at least start by putting low 
power through the filter into a dummy load and tune it for maximum 
output through the filter, then estimate how much attenuation is present 
in the ham band. My transverter/IF combo won't go outside the 902 - 906 
MHz limits, but it might be possible to exceed those limits a bit by 
using my antenna analyzer as an RF source (below 144 and above 148 MHz) 
instead of the IF radio. I'm also unsure how "tight" the filtering of 
the transverter is, and if it would put out much of a signal outside the 
intended frequencies when driven with an out-of-band IF signal.

DCI will make a filter to order, if you have the cash. See:
   http://www.dci.ca

I inquired about a filter with a very narrow passband centered on 902.5 
(to allow for both 902.1 and 903.1 stations) and 3 MHz wide.  They put 
together a simulation of a 6-pole filter, and for such a narrow 
bandpass, you end up with about 2.3 dB of loss at the tuned frequency, 
but this gets you down to -40dB at 898.5 and 906.5 MHz, or -90dB at 
892.5 and 912.5 MHz.

They also simulated a 4-pole filter with the same target passband. It 
has less loss at the midpoint frequency of 902.5 MHz (-1.5 dB or so), 
and less steep skirts (about -28 dB at 898.5 and 906.5, and "only" -60 
dB at 892.5 and 912.5 MHz).

What would these filters cost?

The 4-pole filter in a "low power 3-inch extrusion" is $299 ea, or $259 
in quantity 2+

The 6-pole filter, same style, would be $369 and $329 respectively.

(These prices from their web site pricing page.  See below.)

I didn't ask how low "low power" is, but my 144-146 MHz filter from them 
in the same size handles my 150 watts without problems. If the power 
handling is an issue, you could put the filter on just the receive side 
(assuming your transverter has split RF connections and an external T/R 
relay), but putting the filter at the common antenna connection would 
also help clean up your transmitted signal if it has any noticeable 
spurs on it.

I got a response back from them when I asked if they would consider 
adding a stock filter for the 902 Ham band (they currently have filters 
in stock for 6M through 70cm), and this was the response I got:

> We normally would not stock a filter with a custom band like that. We may be able to get the cost down using a minimal number of cavities. Have a look at the attached simulations and let me know if one of them looks selective enough for you. The 4 pole would be the cheapest way to go. This filter would be made to order, our lead times our typically 2 - 3 weeks. With a multiple unit order we would be able to get the cost a little lower yet.
> 
> Does this filter have to be mounted outdoors? If so that will add a little insertion loss to the graphs I sent as it uses a different type of body.

So, a "group buy" might bring the prices down a bit more.

It took a bit more poking around on thir web site to discover that they 
already have stock filters that cover the entire 902-928 MHz band, in 
the "Wi-Fi / ISM" section of their site rather than the "Amateur" 
section. These are 8-pole, weather-resistant filters. There is one that 
covers 902-928 completely, then an 8 MHz wide model in that range, and a 
5.5 MHz wide model in that range (you pick the center frequency, I 
think, on those). But they are $599 each!

And before we even considered going ahead with such a group order, I'd 
want feedback from folks who know more about this than me if my 
semi-arbitrarily selected specifications are the appropriate ones.  A 
less narrow passband and less steep skirts remove less junk, but also 
provide lower insertion loss at the working frequencies. I don't know 
how much suppression is needed and how close to the working frequency it 
is needed, in order to do the job well.

The folks at DCI make very high quality products, so I have no qualms 
about buying from them. But I guess I'd like to see how well my $30 eBay 
special works before I'd commit myself to buying the custom filter.

Jon, are you aware of other sources for cavity filters? Some of the 
companies that make repeater duplexers have cavity filters that would 
cover the 902 band, but I suspect that the price new is pretty high. If 
you have two, there must be SOME other source out there!

73 de WØJT