Lucky Find! A Kenwood TS-670 Quad Band Transceiver

Hi there! If you reside in Singapore, you would be aware that at Sungei Road, lies a flea market, also known as the “Thieves Market”. Here’s how it looks like:

Taken from Wikipedia article: “Sungei Road”

Well I like to drop by there time to time. Who knows? One might find something quite interesting over there. That was the case for me when I made an interesting find over a month ago.

Being someone who is fascinated by electronics and radio, I would lookout for devices such as valve amplifiers and such. At the time, I was in need of a cheap microwave oven, so that I could re-use the high voltage transformer for a valve amplifier project. I scoured around, looking for one, but I didn’t find one (at least not for cheap! =P). Disappointed, I left the area. But as I did, my eyes lit up! Something interesting caught my attention! What could it be? This:

 

 

 

 

 

 

 

A Kenwood TS-670 Quad Bander! I knew I just had to grab it, and so I did (Impulse buying… That’s why I am broke! XD). The unit is in a very good shape when I got it. For something that could have been lying around in the flea market for at least a few days, it was relatively clean, scratch free and dust free, even inside!

Taking it to Blazer (Radio store), Mr Phua let me use his power supply, and we did the smoke test. It worked! I took it back home and verified that it could receive and transmit. I noticed though, that the dial is a little on the fritz now. I can get the frequency to go up, but when I spin the dial the opposite direction, the frequency still increments! Well, it is a small problem, and I’ll fix it when I have the time.

What surprised me is that the seller provided me with the instruction manual (albeit in Japanese), schematic diagrams and an external SWR/power meter to go along with it. All for $80 SGD. I will probably take it out on field day or one of our QSO nights.

Thanks for reading!

73,
Joyce (9V1AN)

RF Dummy Loads – Observations and experiments

Hi there! I have been busy and haven’t got the chance to post anything for quite a bit. Well, I’d like to share my observations that I made when I was experimenting with RF dummy loads. Loads and impedance matching itself is very interesting, but quite a broad field on their own, so more on that in future articles.

Well, a dummy load is meant to allow transmitters to perform on-air tests, simulating an antenna, without radiating much power out. A dummy load is essentially, made out of resistors. As you know, a perfectly matched dummy load would have its impedance match the transmitter’s. An ideal transmitter for amateur radio would have an output impedance of 50 ohms, with no imaginary component (50 + j0).

I have been asked questions like: “Hey! Can I use wire-wound resistors for use as an RF dummy load?”. So, to answer that, I’ll be showing you my observations with various RF dummy loads. Let us start with a commercial RF dummy load. Pictured here is one that I use for quick tests.

 

 

 

 

 

 

 

 

And here’s the corresponding measurement data:

I’m sorry for the lower resolution on this image. For most of the amateur frequencies, say up to 500 MHz, the VSWR is lower than 1.5:1. That’s quite okay! Now let us look at another load. This time, it was constructed by Mr Jeff, out of axial resistors. Here’s it is:

 

 

 

 

 

 

And of course, the measurement data.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

As you can see, the VSWR is at 3.76:1 on the UHF band. The performance is not as great as the commercial dummy load.

At the start of the year, I asked myself a question: “Why can’t we use wire-wound resistors for a RF dummy load?”. After all, wire wound resistors have high power ratings, and a few could easily dissipate 100 watts from a transmitter. Being curious, I decided to build one myself. Here’s my completed load:

 

 

 

 

 

 

 

 

 

 

And here is the measurement data:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Totally not good! The vector network analyzer doesn’t lie! The load performs very terribly! Not a good idea to begin with. Why is that so?

The name itself tells us why! Wire-wound! You see, they are made by coiling a resistive wire! It behaves like an inductive load! A little computer model shows similar (kind of) results:

 

 

 

 

 

 

 

 

 

Just a tiny, tiny amount of inductance is enough to make the VSWR very high, especially on the VHF and UHF band. Just so you know, this is a simulated sweep from 1 – 500 MHz. Granted, this may not be a very accurate model but it does show that reactances do matter in the realm of RF electronics.

Anyway, hope you enjoyed reading my post!

73,
Joyce (9V1AN)