Hi Skipp,
Took the weekend off, needed to get away from this stuff for a while,
was starting to get dizzy with so much info coming and going all over.
--- In Repeater-Builder@yahoogroups.com, "skipp025" <[EMAIL PROTECTED]> wrote:
>
> You should not be thinking less than 8 poles, maybe even
> more and don't forget the notch filters I mentioned in
> both the tx and rx paths.
We decided to go in steps and see if things work out, and if needed,
go back and add some more filtering. After comparing some factors (DCI
vs. Telewave, delivery time + specs) as a first step we will be
ordering the Telewave TPCP-1546C preselector, it has 6 cavities and
has a pretty good looking skirt (-80db @ 4MHz down). Also wanted to
order a 4-pole TX window filter from DCI but their 4 week delivery
time will not allow us to take it with us on this trip (in about 2
weeks time), so we'll see how things go with just the preselector and
if needed would order additional stuff from DCI. Still nothing written
in stone, also would give consideration to your notch filter suggestion.
Additionally wanted to order a Lo-Pass filter to put at the output of
the Tx combiner, as a "just in case". At least take it with us, try
it, and if there is no difference then leave it out, maybe use another
day somewhere else. It's a long way to the site, and it would not cost
much to order it. How 'bout a Telewave TLF-150 ?
>
> You could (and I would) dial up the skirts on the filter,
> which also increase the insertion loss. Don't fret 1.8 to
> 3 dB insertion loss if it cures your problem and makes the
> system work. You have monster antenna gain anyway.
Tailoring the response is one of the things I wanted to discuss with
Ralph but I haven't heard back from him since I wrote him this morning.
>
> Park (remove from the path) the 2 dB pad, the original
> Sinclair pre-selector unit and possibly the preamp. Depending
> on the specific preamp model type, you might want to replace
> it with something that has more IM3 protection and/or
> less noise.
Will do that, probably step by step and testing after removing each
component to get a feel of any changes. In any case I guess it makes
not much sense to leave in the original preselector anyway.
>
> I mentioned I'm not fond of Chips VHF Preamps simply because
> they are bipolar type (higher noise). He says Phempts and
> GasFets aren't worth the money... I say they are and I mostly
> use them in my vhf systems. Check the noise figure on the
> bipolar preamp models, that should be enough to convince
> you the Phempt or GasFet is worth using at VHF. Much
> of the choice depends on location and the noise floor, but
> I like to go with a winner right out of the staring gate.
I agree with you. If a better (and more expensive) preamp gives you a
better performance, solves a problem, or provides peace of mind for
future site requirements, than spending some extra dollars is
definitively worth every penny.
> I would not put a window filter on the tx side, but you
> could if you want. I would put notch filters in both tx
> and rx paths.
So could you possibly suggest the notch cavities that you would choose
in this case, by brand and model? Maybe the Telewave TWNC-150x series?
What diameter would you suggest (they come in 5, 8 and 10").
> Get over the higher insertion loss issue, it's not that much
> in the real world and you're probably going to end up with
> that much in the DCI and notch filter path for the receiver.
Not really concerned with that, specially in the RX path where one can
make up for it rather easily.
> If I spec'd the above example DCI filter for a system I
> was doing, I have them really tighten up the skirts (per
> my specific needs), which would increase the insertion
> loss near or above 2 dB. Sean was nice enough to run
> some preformance tests on the last 3 UHF 10 pole Window
> Filters they made for me. I had a chance to juggle the
> numbers before the final choices were locked down.
>
> There is a preformance point (on the graph) where you
> trade a little insertion loss for a lot of protection.
> You can easily live with a few dB of insertion loss
> but you can't work without the required protection.
>
> The above mentioned Telewave filter is not what you need.
Pardon my asking, but why are you so sure it won't work? Have you seen
the skirts of this preselector? Looks very close to 80db down at 4 MHz
off center, where our Tx carriers are located. Since it is supposed to
have a 2.5MHz window, whe were going to ask Telewave to reduce it to
just over 1MHz, and "dial up" the skirts as much as possible within
practical limits. As a matter of fact the Telewave 6-cavity
preselector "stock" skirts look quite similar (to me / on paper
anyway) to the 8-pole DCI filter. Anyway, just how much rejection you
reckon we need then, more than 90 or more db's?
> ("these are not the Droids you're looking for... move along")
Aaaah, Star Wars fan. Me too, but not so much of the later episodes.
Nothing like the first one (was it "Episode IV" ?).
>
> > Telewave (posted their full response at the bottom) suggested the
> > TPCP-1544C Preselector, but it seems we could only expect 45-50db
> > isolation with that one. A bit tight, if adequate at all in this
> > case. http://www.telewave.com/pdf/TWDS-2005.pdf
>
> Nothing wrong with the tx part Telewave sold you... by patching
> in a fix on the as-built rx side is not really a workable
> solution at this time.
Again sorry I have to ask, but why not? Please if you could elaborate
a bit more... It's just that those 80db rejection at 4MHz off-center
that the Telewave 6-cav. unit has look pretty enticing to me.
>
> I want DCI to stay in business so I can buy more filters from
> them. The last few years in two-way have been very tough and
> most small shops have gone poof. Companies like Decibel have
> been bought by Andrew and you'll clearly see how they are
> contracting the Decibel product line. I'm happy to report
> 2005 is a very good year for the land mobile radio industry.
Yes, I've also noted that, also Wacom got absorbed by them too, if I'm
not mistaken. You turn around for a moment, and when you look back a
lot has changed. Corporate... whatever, I guess. NOT good.
>
> > BTW, yesterday the Telewave response came in. See bellow.
>
> I read it... Telewave can't do anything but offer to sell you
> something they make. You could make a DCI or better rx-
> preselector from some of their cavities. The most practical
> first pick solution for most people is to get the DCI unit.
Nothing at all against DCI, but only wished Ralph could offer us a
quicker delivery time, I really can't work in those 4 weeks he's
offering now into our schedule. In addition to that, we also have to
consider the shipping from Canada to Miami, and then to Caracas.
> But remember your final tx and rx antenna systems should include
> some notch cavities. Telewave makes good notch cavities and
> they should be able to offer coax T options, which lean the
> series cavity response over.
So you do mean something like the TWNC-150x series? I see these are
available in 5", 8" and 10" diameter. Any suggestion what size would
be more adequate for our needs? Not sure if we would need to have the
steepest skirts (10"), or maybe 8" would be more desirable? So if I
got you right, this would be ONE notch cavity in the Tx path for EACH
of the 6 Rx frequencies?, total 6 cavities in series in T pass
configuration? (this of course would filter out any wide-band Tx noise
on the Rx frequencies, right?
> I did a Cellwave 220 Trunking System a short time back. I could
> scan my drawing and email you a copy of what it took to get
> 5 channels of narrow spaced trunking to play on one antenna.
>
> It's pretty much the same layout as your system, just split
> the tx/rx paths to different antenans.
So I'm guessing ham radio lost some of their spectrum on that band to
commercial applications? I understood that whole band was for ham
operators. Here a portion of it is used to link the FM broadcasting
studios to the transmitters. Not much ham activity going on there.
> Tenga diversión
Thanks. Have fun too.
> cheers,
> skipp
>
> (the weekend is here... "now it's time to go to work")
Yeah, got my rest too. Hope you also had a good weekend.
Alex
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