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Tuesday, September 4, 2012

Construction of OZ2M 70 MHz transverter


In this post, you will find, in a blog-like way, my experience mounting this transverter for the 70 MHz band.

Enjoy!

 

Day -2: Spain authorizes the 70 MHz band

October 24, 2008, around 13 PM. I received a SMS from a friend with this text: Habemus 70MHz. What?? That was an absolutely surprise! Nobody waited for this news, and... in October?? Once at home, I read the full text from the administration. The authorized frequencies are 70.150 and 70.200 MHz with a maximum 12 kHz bandwidth, so this means two mini segments centered at 70.150 and 70.200 +/- 6 kHz. It's not a big segment but it is far better than nothing. Power authorized is 10W EIRP.

So, I started to think about equipment, considering the different options available. Transverter? Modified equipment? Something different?

Day -1: Ordering

After some days, I decided to buy a transverter. It works, and it is usable with almost any equipment (even a cheap CB rig). There are many transverter kits available, so which one to choose? My final decision was this transverter from OZ2M for two reasons: It is cheap and its profits go to support the OZ7IGY beacon. I needed only a couple of e-mails and a Paypal payment and my order was made.

Day 0: The transverter arrives home

 The transverter just out the envelope
 
Only two days after the transverter was sent, it arrived home safely. Inside the envelope, there is a metallic box (the transverter's metallic box). Inside the box, there are all the parts: The PCB, four bags with components, a cooper foil, a small piece of coax, four panel BNC and three pass-thru capacitors. Everything seems to be ok.

Day 1: Mounting the enclosure

The first step is to mount the enclosure. The box comes in four parts. The top and bottom plates and another two plates. They must be soldered together to make the transverter's shield. After soldering, it's a good idea to make the holes for the BNCs and the pass-thru capacitors.


Once you have made the holes, you can solder both plates together to construct the box, and then, solder the BNCs and the pass-thru capacitors. At this point, you have the box completed, so you can install the PCB inside and solder the PCB's grounds at the border to the plates. Use a high power iron to do this.


Congratulations! You have your enclosure finished, so you can start to build the transverter.

Day 2: Mounting the components

I started mounting first the resistors and then, the capacitors. Not too many things to comment out: Just some extra 180 ohm resistors and two missing 27K ones I quickly found in the junk box. No problems with capacitors.


 After resistors and capacitors, I mounted the coils, mixers and the crystal.


And then, I mounted the relay, the diodes, the regulator and in last position, the transistors. I recommend you to mount transistors the last ones. Remember to mount the ferrite beads on Q1 and Q8

The transverter finished
 
The total time to solder all components is about 4 hours.

Day 3: Testing the local oscillator

Just after power up, the local oscillator started to oscillate nicely... so, I connected the frequency meter to R14 using a x10 probe and adjusted L10 - L9 for maximum signal (it's a frequency meter with signal level indicator), and L11 to get the 42.000000 MHz. I advertised L10 also affects the oscillator frequency, so play with L10 and L11 if you can not reach the desired frequency.

Day 4: The receiver chain

Once the local oscillator is running, it's time to adjust the receiver chain. It's an easy task if you have a weak 70 MHz signal source. I used the VHF/UHF walkie to generate a 70.200 MHz signal using its local oscillator. My walkie its a IC-E91, and according to the manual, its intermediate frequency is 61.650 MHz, so to move the walkie's local oscillator to 70.200, I must tune it to 70.200 - 61.650 = 8.550 MHz. Doing this, I have a weak, but perfectly usable signal at the walkie's SMA antenna connector. I connected the walkie's SMA to the transverter's RX input using a pair of 5 dB SMA attenuators. Why an attenuator? Continue reading...


After an entire evening playing with the transverter, I can confirm you can adjust L1-L8 to maximum signal on 28 MHz (this is, maximum conversion gain), but I discovered these hints:
  • L1 adjustment depends on the antenna load, so it is a good idea to put an attenuator between your source and the transverter just to ensure it sees a 50 ohm load. I used a pair of small 5dB SMA attenuators.
  • L2-L3-L4 adjustment interacts between coils, so adjust them repeatedly until you get the maximum signal level. I noticed Q1 oscillate with some core positions, but they are really far from the 70 MHz point, so this is not a problem.
  • L5 does not affect the signal level. Looking at the schematics I can see it is a resonant LC in a very low frequency, very under 28 MHz so... I left the core at the middle position.
  • L6 have veeeery low influence in the signal level. It's difficult to find the maximum signal point, but it is located about at mid position.
  • L7-L8 adjustment interacts a bit between them, so adjust them repeatedly until you get the maximum output level.
If you get a strong signal on 28 MHz maybe you can have problems peaking the coils. Use the receiver attenuators and / or external attenuators. Don't let your S-meter to go over S7 or S9. Reduce the 70 MHz input if you need it (but remember to keep it at 50 ohms).

After the adjustment, I connected the transverter to the external antenna and I was successful receiving some locals, so the adjustment is finished.

Day 5: The transmitter chain

To test and adjust the transmitter chain, you need a power meter capable to measure easily about 200 mW at 70 MHz, and a 1 mW (0 dBm)  signal at 28 MHz. To get the 1 mW signal at 28 MHz I used the HF transceiver at 10W output followed by two 20 dB power attenuators, so the output is exactly one milliwatt.


After applying the 1 mW signal, and tie to ground the transverter PTT input, I got 0.8 mW output at 70 MHz. I only peaked L12-L13-L14-L15 to get maximum output. It's easy and I had no problems at all. With the 0 dBm input at 28 MHz, I can change the output from 35 up to 120 mW on 70 MHz with VR1.

A 125 dB, 85 cm long attenuator used to measure the 2nd harmonic. It is made with four 20 dB power attenuators and about 20 small SMA attenuators

The next step is to adjust L16 for minimum 2nd harmonic signal (at 140 MHz). I connected the walkie to the transverter's output using a 125 dB (and 85 cm long) attenuator, the value needed to get about half scale on the walkie while listening the harmonic. I tuned the walkie to 140.400 and while transmitting with the transverter, I adjusted L16 to get the minimum signal at the walkie. Near the minimum harmonic signal, I had to remove several attenuators. L16 produces a really deep notch.

After this step, the transverter can be considered fully adjusted.

Day 6: Building the oscillator shielding

The last step is to build the oscillator shielding to prevent the 42 MHz signal to reach the final stage and become present on the transverter's output. Just cut some cooper foil and solder it at the right place at both sides of the PCB.

Day 7: Some problems with the local oscillator

After some hours playing with the transverter, I noticed the local oscillator didn´t start some times after power up. I noticed this problem disappeared if I adjust the frequency somewhat below 42 MHz, like for example, 41.999 or 41.997. Adjusting L11 to get 42.000 MHz left the oscillator to an unstable condition.

I solved this issue adding a 33 pF capacitor in series with the crystal, and adjusting L11 to get again the 42.000000 MHz signal. After some tests, I noticed the problem disappeared and the oscillator starts up every time I power it up.


Finally

I'm very happy with this transverter. Works nicely with pretty good sensitivity. The only drawback I noticed is the input power at 28 MHz. 1 mW is not too practical with current transceivers. Something in the 1W range would be much better. Anyway, it's a good investment. I recommend it!

2 comments:

  1. I have been visiting various blogs for SMA Attenuators . I have found your blog to be quite useful. Keep updating your blog with valuable information.

    ReplyDelete
  2. I also discovered that if you adjust the xtal to 42.000 MHz , is not able to start at power-up. Many thanks for sharing your experience with this kit

    ReplyDelete