From the instructions which come with the units:

===== Connection of the preamplifier into the system is quite straightforward. The coaxial cable from the antenna is connected to the preamplifier input terminal (labelled ANT). Preamplifier output (labelled XCVR) feeds the coaxial cable that connects to the station transceiver or amplifier. Control cable used with the MSPXXXVDG should be shielded. Non-shielded cable acts much like an antenna. Nearby lightning strikes are capable of inducing sufficient voltage on this "antenna" to damage the preamplifier. Shielded cable effectively eliminates this problem. Always mount the MSPXXXVDG with the connectors facing down to avoid water accumulation around the connectors.

Wiring of the power connector is illustrated on the schematic diagram. For rf keyed operation only the VDC and GND connections are necessary. The preamplifier will sense the presence ofrfand automatically switch the preamplifier out of circuit. Hard keying of the preamplifier for transmit/receive changeover can also be accomplished. The C terrminal of the power connector places the preamplifier in transmit when grounded. Also, transmit/receive changeover can be controlled with only the +VDC connection. Apply +12VDC for receive and remove for transmit. The dropout delay function should be defeated for hard keying operation. For hard keying operation transmit/receive relay sequencing is mandatory if preamplifier failure is to be avoided. A circuit such as the ARR TRS04VD will provide sequentially keyed outputs that accurately control the timing of all system transmit/receive changeovers. Review the TRS04VD instruction sheet for more detailed information. =====

I OCRed this, so the formatting is not great. My understanding is that "hard keying" they mean something like QSK where you will beat the relays to death. Certainly CW, and perhaps SSB. However, there is a "relay delay dropout capacitor" which as shipped will protect the unit, and of course preclude QSK. So I suspect the answer to your question is no. In my satellite operations, I have always been tranmitting on a different band than receiving, so I just left the pre-amp on the TX side off. No harm if you forget, of course, unless you do it all the time.

Hope this helps,

Alan WA4SCA

On Mar 12 2007, Edward Cole wrote:

At 05:15 AM 3/12/2007, Stargate wrote:

...

Is a sequencer needed when using a 160w switchable preamp from ARR?

Let's analyze this:

You send a voltage to run the preamp. It needs voltage to switch to Tx mode. When RF from your transmitter arrives it switches over to bypass the preamp. I would guess if you turn off the power to the unit, it can't operate correctly, so how is a sequencer going to help? I have not owned one of these units so not familiar with their power wiring. If you have two 12 volt power contacts then you might run the preamp separately from a sequencer.

Sequencers don't have to only apply/remove DC power in a set sequence. In fact, in the general case, a sequencer has a series of switch contacts that are enabled in the proper order and disabled in the opposite order. The switch contacts could be configured as NO (normally open) or NC (normally closed). These can either provide and remove DC power, or short the pre-amp's own PTT circuit to ground to take it out of receive mode (or, depending on the model of preamp, maybe +12 is PTT instead of GND). The Down East Microwave sequencer has a DPDT relay for each of the sequence stages, so you get two independent NO and NC contact sets to play with as you wish.

If (like the ARR models) the preamp has built-in bypass switching, one issue that remains is how quickly and completely the preamp gets switched out of the transmit path. If slow to switch, and/or if the isolation is insufficient, a spike of RF may still enter the very sensitive preamp and fry it. With a sequencer, you can (usually) select whatever switching delay is needed for the isolation relays to reach a fully switched position, and you also get to pick the isolation relays themselves, with attention to power dissipation and degree of isolation needed. I believe that ARR says that the built-in RF-sensing relay is good for up to 160 watts, but a sequencer with a separate isolation relay could basically allow you to use 1.5 kW, if you had an awfully darn good reason to use so much power, ;) that is.

On my 144, 222, and 432 bands, I have an RF Concepts "brick" amplifier with an internal preamp and RF-sensed automatic switching. On 144 and 432, these are "soft keyed" (i.e. keyed by sensing RF). On 222, which is done with a transverter instead of directly out the back of my FT-847, I went to the effort to "hard key" the amp/preamp by using its PTT input (which I had to re-configure as PTT to GND instead of PTT to HI). Both "soft" and "hard" switching methods work OK, but on 144 and 432 it is best to briefly pause after keying the mic before you start talking, to insure that the PA is switched in, and when you unkey, you have to hope that the person on the other end pauses briefly before responding because it takes a noticeable amount of time for the amp/preamp relay to switch back to receive mode. (The delay is intentional, to prevent relay chattering.) On 222, since the radio, transverter, and external PA/preamp are all hard-switched by their own PTT signal, the send/receive cycle is much smoother, no excess delays, no chance of relay chattering if the loudness of your voice drops low for a moment, etc. So I definitely prefer the "hard keyed" method of operation. One of these days, I will get around to modifying the 144 and 432 amps for hard keying also...

On 902, 1296, 2305, and 3456 MHz, my transverters were built with split Tx/Rx, which makes it simpler to connect a preamp (if needed) to the Rx and a power amp to the Tx, and of course you need a T/R coax relay external to the transverter. So far, because my PA's on these bands are low power, I hard-key the T/R relay but it is not sequenced -- radio PTT = transverter PTT = T/R relay PTT, all at the "same" time. When I move up to the high-power class of operation, I will install a sequencer on each. We have local folks here who have had the experience I am trying to avoid, namely simultaneous switching works ok at low power, but add in some REAL output power and things fry if not sequenced. These bands have PA's of 40-120 watts waiting to be integrated. (This is for terrestrial work, not satellites, by the way, as you can probably figure from the high powers and non-satellite bands/sub-bands mentioned...)

73 de W0JT AMSAT-NA LM#2292