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900 Mhz Project

It's really 902-928.

I got my hands on a few Kenwood TK-941 radios to play with. Nice rigs and easy to modify.

Here is a copy of the Service manual for TK-941

Here is a copy of the Operators manual for TK-941

Programming Kenwood TK-941 by W7UVH

Program the radio using the Kenwood program, KPG25D and KPG4 programming cable with all of the ctcss tones you desire and all other functions. Use FCC channel number 200 in all systems. This will indicate frequency 937.5000. Save this file and exit program. Run the program, Kenwood3.EXE. Select the TK-941. Enter the frequency of the FCC channel 200, which you used in the programming. That frequency is 937.5000. Entered, you will see the binary code is 08 9B. Enter "Y" for another frequency then enter a frequency you desire. Write down the 4 digit binary code and continue until you have all of the codes you require for the number of "FCC 200's" you entered in the programming program. Run your Hex Edit program and load the file ***.910 you created when you programmed the TK-941. In lines 20, 120, 220, 320, 420 and every 100 on, you will see the FCC number 200, 937.500, binary number, 08 9B, every where you enter "200". If you have entered 200 as several groups in a system, you will see them in the same line. Now you may start replacing those binary with the one you have written down as the frequencies you want. When complete save as the same name or change as you wish. Exit and reload the Kenwood program and load the program you just saved. Check each system and group to make sure you have the frequency you want. DO NOT attempt to edit the FCC number selection. The result will be that it goes away....and you will have to edit the file again.. Remember that the Frequency you enter will be the Receive frequency and the Transmit will be 39 MHz below that frequency. Not Good! Remember to use T\A in the original programming. You will transmit and receive on the same frequency. All of the transmit frequencies should be programmed into one system, each receive frequency will be an individual systems. Select "Last Used" in the program. Select the transmit system and the repeater input you want, turn on the scan and you are ready to transmit. Let up on PTT and the radio scans to the repeater output. DO NOT transmit while receiving or you will be moved to that frequency. Wait for the repeater to drop and resume scan before transmitting. I have come up with a way to keep from transmitting while receiving but involves pulling the PTT jack, drilling out the PTT ground, reinstalling it and adding a 2N2222 to keep collector high while receiving.

If you want the radio to receive great, below 926.000 Mhz you will need to replace the filters. DigiKey Part Number: TKS2617CT-ND. These are made by Toko and the Part number is 4DFB-915E-10. Here is a copy of the data sheet.

This is a image of what needs to be done.
click for larger image

This is how I replaced the filters in one of my TK-941 radio:

First remove the screws from the top and bottom covers.

Remove the face plate, remove the small screws holding the front PCB.

Remove the front PCB. (This gets the ribbon cable out of the way.

Remove the flat plastic wire harness from the front PCB and main PCB

Remove all the rest of the screws! all of them...

Unsolder the coax from the main PCB.

(Click to enlarge)

Cut the felt from the clip.

Remove all the clips.

Gently lift the main board out of the chassis.

Now the fun part, I don't have the right equipment to to this job. One would be better served taking to a shop that did surface mount work. Do this at you own risk!!

I used wick to remove the solder from the two small wires and then lifted these clear of the PCB

Then I used a large iron to heat the whole filter until the solder was hot and melted.

(Click to enlarge)

Then I quickly removed the filter with needle nosed pliers. Tricky is the word!

Next I cleaned up the mess with some wick and flowed fresh solder on the pads.

I had to form (bend) the ears on the replacement filters so the would reach the pads on the PCB. Be gentle!

(Click to enlarge)

I then tacked the filter in place using the ground ears first. Then finished with the others. Be sure to check for shorts. The ears are real close to ground.

(Click to enlarge)

Now put it all back together and hope the smoke stays inside the chips.

Ronn

Here is a mod to prevent keying the radio while the repeater is still transmitting.

900 MHz 9 Element Plumber’s Delight Yagi Antenna

Picture of 9 element 900MHz antenna

NOTE: This antenna differs in element mounting, in that elements are solid rods and mounted through the boom. Refer to other areas of the Antenna area for construction of the mounting plate. These are common to all my antennas and so are only described once.

All rods used as elements and the aluminum boom can be cut with a sharp hacksaw.

All aluminum that I used is T6 or T5 hardness.

I purchased my aluminum at Ridalco, 1551 Michael Street (near St.Laurent and Belfast). Total aluminum cost was under $25 (incl tax).

This is a fairly straight forward antenna to make, if you have the tools and you measure, mark, and cut very carefully. It does take a little time but it's is a fun project.

Boom length is 71cm or about 28". I actually used 30".

Boom is made from 3/4" T5 square aluminum tube. T5 is more than adequate for this short and light weight antenna.

Elements are 3/8" T6 round aluminum rod. You will need approximately 48" of rod.

Element dimensions and Spacing (all in cm)

Element Name	Element Length	From Front	From Last Element
D7	11.65	3.00	-
D6	11.8	9.70	6.70
D5	12.10	16.40	6.70
D4	12.20	22.50	6.10
D3	12.70	28.70	6.20
D2	12.10	34.80	6.10
D1	12.40	40.80	6.00
Driven	14.60	46.25	5.45
Reflector	17.10	53.80	7.55

Mounting the Elements

The elements are mounted thru the boom. Drill the boom carefully using a squared up drill press. Use a grinder to lightly dress up the edges of the elements or they will not fit into the hole in the boom. Once inserted into the boom, carefully centre the element. Drill a small 7/64" hole thru the boom and into the element, deep enough to accept the screw. Using a tap set, tap the boom and element to accept a 6-32 stainless steel screw. I cut the screws to 5/16" length. Carefully insert the 6-32 screw into the newly tapped hole. Tighten but be careful not to over tighten they snap.

Note: When you are drilling and tapping the holes to accept the screws, if you choose not to drill all the way through the element, be sure that you allow for the nose of the tap. Be careful not to force the tap too deep or it will break off and you will have to build another element.

Antenna Interfacing

NOTE: Fabrication of the gamma tube arrangement and mounting of the N-type connector can be a slow process - work carefully, be patient, and it will probably work fine.

This antenna uses a gamma match. The gamma tube is exactly 1" centre-to-centre (ctc) below the driven element. The tube is 4.3cm long and nicely accepts the gamma element. The gamma rod (which actually forms a capacitor in series between the tube and the connector) is precisely 2.2cm long, from the centre of the hole (which connects to the N-type connector) to the end (however I used 4.4cm from RG8, because my centre conductor is smaller diameter, hence the capacitance would be smaller). The gamma rod was originally 3/16"d or .5cm (again - the diameter of mine is smaller than the original). The gamma rod is insulated from the gamma tube by a dialectric which nicely fits inside the tub and which accepts the gamma rod. In my situation the dialectric was actually the material around the centre conductor of RG8 coax.

900 Mhz gamma match

The gamma tube is held to the element by a mounting plate which starts out as a piece of 1/4" thick aluminum flat plate, 3/4" wide. Measure 3/8" from the end of the plate and at the mid-width of the plate place a dimple for drilling a 3/8" hole. This will eventually slip over the element and hold the gamma tube at precisely 1" from the element.

For ease of handling, the gamma mounting plate should not be cut off from the aluminum stock until all other work with it is complete.

At 1" from the dimple for the hole for the element, place another dimple for the hole which will hold the gamma tube.

The gamma mounting plate, element and gamma tube must be held rigid. Rigidity is accomplished by installing 2 set screws: one for the gamma tube and one for the element. Using a drilling clamp and drill press, carefully drill a set screw hole through the 1/4" aluminum until it intersects the hole for the element. Do the same for the gamma tube hole. Tap the 2 set screw holes. I used 4-40 hex head screws that I bought at the hobby shop for $1.79 (for 4).You may have to touch up the holes for the gamma tube and the element as it does not take much to distort these holes and make the fit difficult. Try using a fine round file, or fine metal sandpaper wrapped around a drill bit, if the fit is too tight to work with. Ideally you want a snug but workable fit between the mounting plate, tube and driven element.

Once all the drilling is completed, you can cut off this mounting plate with a hacksaw, at exactly 1.75" from the end.

The gamma tube protrudes (is tightened so that it protrudes) 9mm or .9cm from the mounting plate, towards the connector.

The gamma mounting plate is fastened on the driven element, 2.9cm ctc from the boom, for starters.

Mounting the N-type connector

This is the messiest part of the construction. I think the easiest approach would be to start with some 1/16th inch thick, 2" angle aluminum. Mark things out, do the drilling , then cut it down to size. I fastened the mount to the boom with 2 pop rivets and then fastened the connector to the mount with 4 small machine screws.

click on photo for larger image
NO5X's Version

Closing Comments:

This antenna is short and light. Considering its frequency, you should end-mount it. You can use exactly the same boom-to-mast mounting arrangement as for centre mounting.

This antenna should look like an electrical open, when you measure across the feedline.

This design is cloned from a commercial antenna which is robust, and has a broad bandwidth. Tuning range is approximately 900-950MHz and the gain for 9 elements is 10dBd.

Not field tested yet but I don't expect any problems. It tuned up in the basement in a few minutes.

Frequency/SWR curve supplied by Gary N1TLL

Frequency/SWR curve of 9 element 900MHz antenna

More help with programming!!!!

The following is for the 941, however you should be able to program the 940
as a 941. The VCO will need to be adjusted to lock at the highest receive
frequency. N2MCI has a program KW900EZP.EXE that is easier and might work if you
program the radio as a 941, saved as ???.910 Programming Kenwood TK-941 by W7UVH

The two ceramic RF filters will need to be changed. May still be available
from DigiKey (TOKO) Program the radio using the Kenwood program, KPG25D and KPG4 programming cable with all of the ctcss tones you desire and all other functions. Use FCC channel number 200 in all systems. This will indicate frequency
937.5000. Save this file and exit program.

Run the program, Kenwood3.EXE. Select the TK-941. Enter the frequency of the FCC channel 200, which you used in the programming. That frequency is 937.5000. Entered, you will see the binary code is 08 9B. Enter "Y" for another frequency then enter a frequency you desire. Write down the 4 digit binary code and continue until you have all of the codes you require for the number of "FCC 200's" you entered in the programming program.

Run your Hex Edit program and load the file ***.910 you created when you
programmed the TK-941.

In lines 20, 120, 220, 320, 420 and every 100 on, you will see the FCC number 200, 937.500, binary number, 08 9B, every where you enter "200". If you have entered 200 as several groups in a system, you will see them in the same line. Now you may start replacing those binary with the one you have written down as the frequencies you want. When complete save as the same name or change as you wish. Exit and reload the Kenwood program and load the program you just saved. Check each system and group to make sure you have the frequency you want.

DO NOT attempt to edit the FCC number selection. The result will be that it goes away....and you will have to edit the file again..

Remember that the Frequency you enter will be the Receive frequency and the Transmit will be 39 MHz below that frequency. Not Good! Remember to use T\A in the original programming.

You will transmit and receive on the same frequency. All of the transmit frequencies should be programmed into one system, each receive frequency will be an individual systems. Select "Last Used" in the program. Select the transmit system and the repeater input you want, turn on the scan and you are ready to transmit. Let up on PTT and the radio scans to the repeater output. DO NOT transmit while receiving or you will be moved to that frequency. Wait for the repeater to drop and resume scan before transmitting.

I have come up with a way to keep from transmitting while receiving but involves pulling the PTT jack, drilling out the PTT ground, reinstalling it and adding a 2N2222 to keep collector high while receiving.

Gene W7UVH

Click here for more programming help.

900 Mhz Ham band and the Motorola GTX 900

I Picked up 2 of these GTX 900 off eBay. They are easy to reprogram.

Here is a photo of the programmer cable I used.

For loads of Moto info go here!

For GTX programming info go here!