Buz

+12V

ation of about a second or so.

fn the release cycle (receive), this circuit provides a one-second time constant. The diode is effectively out of the circuit at this time. This release time constant is sufficient to hotd the circuit operational between SSB modulations, to prevent premature release of the keying relays, if you want a longer time, increase the capacitor value. Adding an additional 1 jiF capacitor (now 2 |iF total) gives a two-second hold time,

Two FETs were used in our circuit: one for the receive circuits and the other for the transmit circuits. The speed of operation for each circuit is determined by the R/C time constants on the FET's inputs. The hold time or hang time after releasing is controlled by diode steering of the exact opposite timing to the other FET's gate circuit accomplished by the RC charging time constants of a 1k resistor and 1 jiF capacitor vs. a 47k resistor and 1 pF capacitor. That controls the key-up time control. For the circuit to meet design constraints, the exact reverse of each circuit is needed. This was accomplished by using diode logic to take the voltage available on the opposite charged capacitor to control the other FET's gate. See Figure 3, the FET switching delay circuit.

The switching relays used for control were connected to ground In each of the two FET source leads. The drains were tied to Vcc (+12 volts), As long as the op amp is off (no RF detected), the relays are in a released condition. The relays used for control voltage are not special relays, A suitable relay for 12 volt operation can be purchased from Radio Shack (PN 275-217; $5.99 each). Any low current 12 volt, multicontact relay can be substituted and should work well. I scrounged my relays off of some |unk PC boards and saved a few bucks.

How do you unsolder multicontact items from PC boards? Well, get down to your local hardware store and purchase a paint-stripping heat gun. It un-solders better than it strips old paint, and it's inexpensive, less than $20.

Putting the circuit to use is the custom part of the exercise. I will describe my application, but your circuit wilt vary, depending on how you switch your rig's elements about. In my transceiver, I switch my preamplifier forward and in reverse, using it for receive and transmit. Additionally, In transmit I switch the now reversed preamp to drive the TWT amplifier to full power output. To accomplish this switching f use a bank of four SPDT coaxial relays. Another relay is used to switch out a direct connection from the mixer JF port to the NT and replace it with a 10 dB attenuator for transmitting. The third switch point is internal to the TWT power supply to transfer it from standby to transmit.

My sequence of events is as follows: I want the four relays to actuate first, along with the IF switching relay. Then and only then do I want the TWT relay to actuate. In this way I

protect the low-noise preamplifier from the transmitter power stages by keying the power amplifier last when going to transmit. In the power down sequence this same relay must release first. This switching sequence will be similar in your situation, depending on what constitutes your elements. !n any case, to set up your system's switching requirements, all you need to know is which relay is fast to operate and slow to release. See Figure 4 for the full system schematic diagram.

In actual practice, both our rigs have the circuit constructed dead bug fashion on a scrap piece of copper PC board, The FETs I used were BUZ-72 types, capable of several amps of switching capacity for relay drivers. Quite an overkill, but they should work for quite a while without problems,

I plan to make artwork for this PC board in the future and will put it in the column in another issue.

West Coast VHF/UHF Conference

The West Coast VHF/UHF confer ence will be held in Ventura, California, on the weekend of May 12 through 14th, at the Ventura Holiday Inn, 450 East Harbor Boulevard. Planned events include technical talks and vendor exhibits, and a Saturday banquet and Sunday breakfast, both with guest speakers. On Sunday, antenna range capabilities with pattern plots will be calculated. A color plotter will make plots, labeling each with antenna description and 3 dB beamwidth. Coverage will include 2 meters through 10 GHz, The test range, 300 feet long, will be located on top of the parking garage next to the hotel.

The noise figure measurements, a very popular test session, will also have plotting of your VHF and UHF converters and preamps. This is a tradition deep-rooted in the many conferences that have been held over the years. Hewlett Packard will provide the latest noise figure meter, which will be integrated with a computer and a graphics printer. For anyone who intends to bring preamps, standard con nectors are a necessity. They should be female SMAh N, BMC or UHF to allow measurements to be made. Please provide clearly marked power terminals that can be test-clipped for the test measurement, I would add, mark the power pins clearly, least you have a 90 dB attenuator.

Registration for the conference is $15, due by May 7P for pro-registration awards. For information, contact the Venture County Amateur Radio Club, VCARC PO Box 2103, Oxnard CA 93033, or Steve Noll WA6EJO at (805) 647-4294. For exhibit space, Cail (80S) 264-1978,

As Is normal practice, the ARRL will publish the proceedings for the VHF/UHF conference and will make them available at the conference for S10. After the conference they will be available from the ARRL directly for $10 plus appropriate shipping.

Well, that's ii for this month. As always, J will be glad to answer any questions related to VHF and UHF. For a prompt response please send an SASE. 73 Chuck WBGIGP,

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