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June 20th 04, 11:21 PM
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In article , Paul Burridge
writes: On 18 Jun 2004 04:36:49 GMT, (Avery Fineman) wrote: Putting on the skeptic's hat, I'd have to ask why the need of a preamp? If more receive gain is desired, the safer place to do that could be in the transceiver itself. The T/R switching is already there and presumably safe to use as it is. An exceedingly well-written explanation, if I may say so, Len. You're of course right to suggest the above solution; the only problem nowadays being the mind-boggling lack of space inside the new transceivers! As for the use of switching relays at RF; surely someone must make RF relays specifically to preserve the 50 ohm characteristic impedance of most transmission lines? After all, 29Mhz is one thing, but what is one to do at say 2Ghz?? Thank you, Paul. There are many, many UHF-and-up relays available on the market, but at a price most of us can't afford for hobby projects. Those designs, as with UHF+ connectors, are arranged to minimize the literal discontinuities that change spacing, dielectric material, and so forth that alter the "characteristic impedance" of the equivalent transmission line section of the structure. That's the price one pays. There are some alternatives at lower powers. For an example, reed relays can be fitted in brass sleeves (non-magnetic anything) and form an acceptible coaxial transmission line section. Takes a bit of experimentation to get the "outer conductor" v. inner conductor ratios correct. I know those can operate to 3 GHz with less than 1.5 VSWR. A high-environment relay (such as Military airborne) is the "rotary" type. The magnetic core is attached to a plastic spider, all arranged to move the spider in a short arc. Fingers on the spider move low-length contacts welded to the hermetic seal pins of the cylindrical case bottom. Moving contact length is roughly a quarter inch, less than a cm. Very low contact-to-case-ground capacitance, under 2 pFd, any combination. Allied Signal made thousands of them and that style is still available, but for a horrendous price, such as US$26 for a 6PDT, new. 2 A, 28 VDC contact ratings...usually with a 28 VDC coil, but there are variants of that for new designs. Hermetically sealed, those last forever. I have some factory surplus left over from 1957, used in Hughes Aircraft radar sets. Those can be found in various surplus gear and most hobbyists just discard them, thinking they can't use them for anything practical. I've had success using those in RF switching up to 1 GHz with VSWRs less than 1.8:1. A variation is the "crystal can" low-power relay, again sealed, but contacts seldom go beyond 2PDT. Different contact-move arrangements but the contact lead length is very short, minimum inductance, and very low capacitance to case ground or other contacts. Those have cases the size of an HC-6 quartz crystal unit, some slightly larger but in the same dimension ratio. Even the open-frame, leaf-spring contact arrangement can be used at UHF with minimum discontinuity for balanced line switching. Those require much examination as to the relative spacings of the contact leaf springs, leaf widths, etc., and some are the wrong dimensions and cannot be used. That kind of relay was never designed for RF applications so there are no manufacturer's ratings for that...one has to experiment. For very quick checking of what can work and what can't, a TDR or Time Domain Reflectometer can show that immediately. TDRs have been around for three decades but are still horribly expensive. Those who have them at work have an advantage of sneaking in experimentation during lunch hour. :-) |
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