Build a 432 MHz Beacon! Tom Haddon (K5VH)


One of the major goals of the Roadrunners Microwave Group from the beginning was to implement a beacon program here in south Texas. The club now has a dozen beacons on the microwave bands, 902 through 10 GHz. It became clear, however, that beacons for the lower bands were also necessary.


Introduction:

A "beacon" in the sense used here is simply a low power transmitter the periodically transmits on some established frequency. Its purposes are to aid in assessing signal propagation and to serve as a known distant signal for testing receivers.

There is no fixed method of creating such a beacon, and this article explores how I happened to go about that process.


General Beacon Requirements:

Here are some of my own requirments for a beacon:

  1. Reasonably stable CW signal source (< 5 KHz drift on a typical 105 F Texas day).
  2. Power level minimum 1W, maximum 10W
  3. 12-15V power source
  4. Modulation either FSK (low power level) or OOK (at high power, 10W)
  5. Programmable keyer, auto-recoverable after power loss.

A compact signal source was not readily available for purchase. Ideas of using modified FM transmitters just did not seem useful: they were neither compact or stable enough.  However, it seemed that the 1296 Weak Signal Source (WSS) from Down East Microwave could be readily modified for this chore. This can be purchased as a board kit, and is easily modified for work at 1/3 frequency.

To put the beacon together, several components were assembled, mostly from Down East Microwave (see Fig. 1):


Component Choices:

Here are the final choices made:

432 Source:   DEMI 1296 WSS, modified
Amplifier:   DEMI 7010 PA
Keyer:   ESS FB-1, modified
Crystal:   Chosen for 432 beacon band, 432.300 - 432.400 MHz
Enclosure:   (See discussion.)

Figure 2 shows the DEMI 1296 WSS block diagram. This source just takes the 12th harmonic of a 108 MHz oscillator; it can also serve as a source for 432/ 4th harmonic by simply changing the filters FL-1 and FL-2. DEMI has TOKO helical filters which will serve fine for this purpose. Existing three MMICs (IC-1, 2, 3) are used in place.


Filter Modification:

The supplied filter must be modified as follows:

  1. If kit version, assemble and test entire board but do not install optional components D2 and L5.  Install new crystal for selected frequency.
  2. Scratch out the input filter connections (at C13 and output of IC3) on the PC board, leaving enough to provide a solder pad for later connection. See Fig. 3 for DEMI layout.

Be sure to provide a power connection to the MMIC IC3 via R10.

  1. Scratch out the output connections from FL-1 and FL-2 also. Leave the stripline connection between IC2 and IC3 alone!
  2. Prepare two 432 MHz filters using TOKO 433-3 filters (or equivalent) on a very small perf board. Use stiff wire for input, output, and two ground leads from each filter.
  3. Mount filters just above old filter areas. Make sure connections are solid; wire should be short and stiff enough to support board without standoffs.
  4. Re-test this source. Output should be stronger than the 1296 unit by about 6 dB; I measured 15 mW (+12 dBm) from mine. Signal was strong, clean, and stable.
  5. Mount the new source into an enclosure of your choice. I used an enclosure I already had from DEMI, but should have used a larger one to allow room also for the FB-1 keyer. Do not mount in the same housing with the amplifier. A picture of the first prototype modification is shown in Figure 4.

Figure 4. Prototype Modification

Because the enclosure may act as a resonant cavity at 432, some RF absorbant material may be helpful; I taped a square to the lid.


The 70 cm 10W Amplifier:

I selected the DEMI 7010PA amplifier for use with this source because it is a good match for the source output power and it's compact size. It uses the Mitsubishi RA13H40447M CMOS hybrid, providing 10+ watts output for 5 - 20 mW drive at 13.8VDC. No modifications are necessary for this amp.

It is a good match for the new source, but care should be given to isolating this high gain device from the oscillator source and keyer (as I found out!). Be sure to solder the brass cover to ground carefully. I bought the board kit without the enclosure and heatsink, providing my own. I mounted this on top of the source enclosure, providing my own heatsink and two fans. In retrospect, I should have bought their enclosure and heatsink to keep down the stray RF.

Use the TXON pin (low current bias, 6 - 15 VDC) to key the amplifier directly. Install a feed-through cap for this, if not supplied.


The Keyer:

The FB-1 kit from Expanded Spectrum Systems provides a PIC programmable keyer in a small package, suitable for beacon projects. It provides two controllable outputs which switch a 2N2222 open collector to ground. To key the DEMI amplifier, additional control circuitry must provide a switched high voltage for the amplifier bias pin. A PNP switcher added to the keyed output of the FB-1 will do the job. See Fig 5.

For complete notes on constructing and programming the FB-1 keyer, refer to the ESS website (www.expandedspectrumsystems.com) and download their manual. Programming is convenient, and only requires a simple connection to your PC or Mac's serial port and a basic communications program like HyperTerminal.


The Crystal:

The crystal that comes with the 1296 WSS is a 108.0083 MHz HC49/U device which results in a frequency of 432.033 MHz (right in the EME band). Useful for testing or a weak signal source, but not as a beacon. So I ordered three new crystals from International Crystal which put this source in the 432.300 to 432.400 MHz range. I sent the original crystal to them so they could provide an exact match to original specs. You can reference my order if you want: catalog item # 473560, ICM order # 91655, customer ID 4047 (Thomas Haddon).

You may want to put an attenuator between the source and amplifier, so not to overdrive the amp. I chose a 2 dB pad for this. Figure 6 shows a picture of the completed unit.

Have fun with this unit - if you get one on the air, be sure to publish it's freq, callsign, and location for us. And Steve Costro.. how about making this easier for us?? - HI!


Parts List (Note 1):

WSS1296K (1) 1296 Signal Source, PC Board Kit $60.00

TOKO433-3 (2) 3 Pole 70 cm Filter ($17 ea) $34.00

BEAD1 (1) Ferrite Beads (10 pack) $1.00

MPS51W (1) PNP Switching Transistor, 1A $0.50

7010PAK (1) 70 cm, 10W Pwr Amp kit, 50 mW input $62.00 (note 2)

CTPS113 (1) Cast Aluminum Enclosure, 4.53 x 3.54 x 2.17 $15.00 (note 3)

Expanded Spectrum Systems (note 4)

FB-1 (1) Freakin Beacon Kit $30.00

International Crystal (note 5)

473560 (1) 108.1xxx MHz HC49/U Crystal $31.00 (note 6)

Total: $207.50


Misc from Junque Box:

Fan(s) for PA

Perforated board for filters, PNP switch

Bias resistors, bypass caps, ferrite beads, as required

RF absorbant material


Notes:

  1. www.downeastmicrowave.com
  2. Or purchase their complete kit for $112.00 - includes heatsink and enclosure.
  3. Large enough for source board, with trimming. Use a larger enclosure to house the keyer also.
  4. www.expandedspectrumsystems.com
  5. www.icmfg.com
  6. Approx unit price when ordered as a group of three. Catalog number 473560.

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