> "vintageaudio2004" <[EMAIL PROTECTED]> wrote: > Ralph from DCI suggested either a 4-pole or an 8-pole > filter. Here's a link he sent, and I must say that it > looks pretty good.
> http://www.dci.ca/html_commercial/graph_158-4-8_wo219.htm 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. > only 1db insertion loss. Price is quite reasonable, only > delivery of 4 weeks (or maybe less), is a bit of a problem > as we ideally would need to return to the site in about > two weeks time. 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. > In regards to the added insertion loss of 1db of the filter, > to compensate for this maybe we could probably bypass the > stock Preselector that came with the multicoupler, and/or > just remove a 2db pad that is now installed between the > output of the multicoupler preamplifier and the power divider. > Would you say this is a viable option? 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. 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. Chip Angle makes great stuff, I use his UHF Preamps quite a bit, but at VHF I use my own Phempt design or an ARR (or other brand, same type) GasFet. > Using the 8-pole might probably seem like a bit of overkill, > but given the critical need to not only solve the problem, > plus the difficult access (it's very distant) to the site > where the system is installed, we would like to have the peace > of mind of an additional margin of safety. For this reason > maybe we might also consider filtering the TX path as well, > also with another 8-pole filter, or maybe just 4-poles. The DCI 8 pole size filter is a min must use size... maybe more in problematic systems. You really need the protection. 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. > The only other Preselector option that we are considering > at the moment is the Telewave TPCP-1546C 6-cavity Preselector, > as it seems to offer between -70 to -80db isolation at > 4-5MHz off center, although we don't like much the 2.5db > added insertion loss. If you care to take a > look at it, the datasheet is here: > http://www.telewave.com/pdf/TWDS-2006.pdf 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. 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. ("these are not the Droids you're looking for... move along") > 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. > > My VHF rx pre-selection system is a very special network made > > for use with non-sequencial (mixed spaced) receiver inputs. Oh > > the joy it was to make... > > I can imagine it was a bit of a pain to make, but glad to hear is > humming along nicely now. both your positive comments on DCI, and > their prompt and helpfull responses actually made me want to include > them in our short list of possible solution providers. You might be > able to ask for a possible sales commission... ;-) 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. > We have that problem here in Caracas. VHF is basically so > congested that it is almost unusable at any of the good > mountain top sites. Entiendo, Que lastima' > 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. 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. 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. Tenga diversión cheers, skipp (the weekend is here... "now it's time to go to work") Yahoo! 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