Saturday, March 19, 2011

75Ohms Coax update

Low loss & cheap, that was just one part of the story about RG-6 sat-coax. The other side would be the (cheap) accessories one can get for the 75Ohms F-system. My secondary grabber system (Target HF3 & Yaesu FRT-7700) is hooked up to 75Ohms for a couple of days. Additionally I am using one of those ("Sat-Flex"-adapter):

For the readers who have never seen those before, such devices are use to lead a TV-sat signal from the LNC to the indoors sat-receiver. Those adapters are squeezed between a window and a window frame.

There are other accessories available at very very low prices. Splitters, barrels, splitters, etc.

And even better news. I not buying the cheapest available, one may be lucky and obtain this:
Well, it was slightly more expensive (€8.- for 10m @ HEMA) than the previous example, and it did not come with F-connectors. However, see the air-cells? Next to the Al-foil shield, there is a Cu-braid, the soul is made from Cu too. This cable can be soldered just fine.
Somehow the stuff reminds me of aircom-plus (€3.35 for 1m)... many times cheaper though...

Monday, March 14, 2011

75Ohms, why not?!

I have been experimenting a bit with more or less cheap (inexpensive that is) materials lately. Came across something called RG-6, or, in other words, very inexpensive 75Ohms coax-cable for TV-satellite reception.
TV-sat-RX means, that losses at frequencies between 1GHz and 2GHz are acceptable. The cable itself proofs on that point, foam dielectric....
We amateurs are using stuff called RG-58, RG-213 etc. for ages.... This relatively expensive stuff is rated 50 (or 52) Ohms, as most of our transceiver's aerial connectors. Most of the modern times transceivers however still employ so called UHF-connectors... something sooo outdated, you would not believe it! And further, look at the inside of 'm transceivers, there is not 50Ohms match found anywhere.... So, what is that hype about 50Ohms anyway?! I don't know!
And hence, I don't care!
However, nowadays UHF-connector have a thread for the the shield. The soul of the sat-coax is copper and hence can be soldered easily. There is hence an easy way to adapt 50Ohms PL-connectors to 75Ohms F-connectors.

F-connector 75Ohms coax and a 50Ohms UHF-connector

My local hardware store provides me with satellite-grade 75Ohms cable for a price of about €5.- per 10m including two of the so called F-connectors... a price, RG-58 cannot match, not to speak about the (two) UHF-connectors aka PL259.
Let's further search for justification of using more expensive 50Ohms-stuff rather than 75Ohms-mass-ware. A dipole is mentioned to have an impedance of 60Ohms.... OK 50Ohms is marginally closer to that than 75Ohms would be.... but... does that matter? I don't think so!
Let's turn to the connectors for a change. I mentioned UHF-connector (aka PL-connectors) before. To the time these connectors have been invented, names like UHF were justified, I believe. However, in modern times, this merely reflects a relict from the glorious past... UHF connectors are considered good for shortwave and lower frequencies. Connectors considered suitable for UHF as of today would be BNC and N-connectors (both 50Ohms nevertheless).
Still 50Ohms, but why? I believe, we stick to 50Ohms (or 52Ohms) for reasons of tradition. Industry seems to like that, since there still is a reason to produce (and sell) 50Ohms coax like RG-58 and RG-213.
As radio-amateur, or HAM for good measures, you always want to watch your options.
I watched mine, and I came to the conclusion that, where ever I would need coax-cable the cheapest commercial (i.e. mass market) solution available was the best for the purpose. Hence I will use RG-6 or whatever (low loss) will be available for cheap at my local hardware store. Who needs specialized retailers, if a tiny change in the setup will do?
Up to now I was writing about low power and low-noise reception. Let's face the other option for a change... QRO! When facing high power, I would anyway not consider using coax-cables. Those just get hot on losses. For QRO, I personally would/will/do go for open wire feed-lines. If that appears too difficult to do, I still would propose using high ohmic window-line for the purpose.
Conclusion: use cheap satellite-TV-coax and forget about expensive 50Ohms stuff!

Friday, March 4, 2011


This is an extremely easy one. Most QRSS operators once built a 30m MEPT. There is an easy way to recycle those transmitters for 6m. With a converter circuit running a 40MHz oscillator, a 10.140MHz MEPT would be right away transmitting on 50.140MHz. Not too bad, I figure. A NE612 would probably be the easiest and cheapest way to get there this way.
Alternatively, a subharmonic mixer (two anti-parallel diodes) on a 20MHz oscillator would be even easier to build.
There are canned oscillators for 40MHz and 20MHz available, not to mention those cheap crystals.

Wednesday, March 2, 2011


Something I was thinking of occasionally, QRSS on VHF, UHF, SHF and EHF bands.

Lets first start with a band that is accessible to all of us, the 2m band.

A short chat with Dave (G7UVW) brought up two frequencies of interest.
  • 144.070 MHz
  • 144.4905 MHz (WSPR)

Both QRGs can be reached with CB-TX-crystals on their fundamental frequency and doubling the signals a couple of times. 9.x MHz will have to be multiplied by a factor of 16. With a little pull and the right oscillator circuit, this all should be fairely easy.

144.070 MHz
Channel 5T (27.015MHz) will be, very obviously, be 9.005MHz on the fundamental frequency. The resulting frequency on the 2m would be 144.080MHz. A little harmless down pull to 9.004375MHz on the fundamental will get us to 144.070MHz.

144.4905 MHz (WSPR-QRG)
We got two options here,  channels 11T (27.085MHz) and 12T (27.105MHz).
11T would require a Pierce oscillator which oscillates above the series frequency of the crystal. The fundamental of 11T is 9.028333MHz, while the required frequency for the target-frequency would be 9.03065625MHz. The difference of 2.323kHz is not problem to a Pierce oscillator at all.
12T will provide a 9.035MHz signal. The pull of 4.344kHz to 9.03065625MHz is doable, could however result in a less stable oscillator.

Put the design into practice seem not to be a great deal, however, frequency stability is a great deal here. So, I figure, good temperature stabilization is in order. The rest is in fact pretty straight forward. I do have to admit that the first ever ham-radio transmitter I built (age 15, just licensed) was a 144MHz A1A TX, based on a CB crystal and BF199 transistors.