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 something sooo outdated, you would not believe it!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!
73!

10 comments:

  1. Hello Joachim, I have always been told that our modern time transceivers would give max. power at 50 Ohm. Now your telling us that there is no matching network whatsoever in there. I never tested 75 Ohm coax on my transceiver to see if it is true why would I? Are they telling us stories just to sell expensive 50 Ohm coax, I think that is what you're telling us? I can't imagine that is true. But certainly I understand that it would be easy to make a transmitter so that it gives max. power at 75 Ohm, so we can use cheap bulk TV coax cable. 73, Bas

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  2. Hi Bas,
    very short note here... I have to leave this computer in some minutes...
    Many modern transceivers have built-in ATUs, they could easily match 75Ohms. Regarding my old transmitter (Drake TR4), had load and plate capacitors that could do the same.
    Sat-grade (!) cable, having about the same diameter as RG-58, should have less losses tha RG-58, in particular since it is supposed transport 1Ghz-2GHz signals over substantial distances in our homes...
    RX-wise, it works perfectly, TX-wise, I will give it a try and post some about it later.
    73! Joachim

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  3. Zie bijv:
    http://www.microwaves101.com/encyclopedia/why50ohms.cfm

    Volgens dit artikel is 50 ohm een compromis tussen 33 ohm (max pwr handling) en 77 ohm (laagste verliezen).

    Er zijn meer van dit soort artikelen te vinden. Google op " why 50 ohms ".

    73
    Ron
    PA2RF

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  4. Google eens op " why 50 ohms ". One of the links tells us 50 ohms is a compromise between 33 (best pwr handling) en 75 ohms (lowest losses) :

    http://www.microwaves101.com/encyclopedia/why50ohms.cfm

    73 Ron
    PA2RF

    ReplyDelete
  5. Google eens op " why 50 ohms ". One of the links tells us 50 ohms is a compromise between 33 (best pwr handling) en 75 ohms (lowest losses) :

    http://www.microwaves101.com/encyclopedia/why50ohms.cfm

    73 Ron
    PA2RF

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  6. Ron,

    thanks for the link!

    As you may have guessed, this rather a rhetorical question ;-) In particular since I am mainly busy with QRP, low loss seems desirable to me.
    BTW, the stuff I bought comprises a foam-dielectric and double shielding with Al-foil and some braid... much more than one could expect from RG-58.
    On top of that, the very cheap price.
    To me, the stuff ideally fits the QRSS-spirit.

    73, Joachim

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  7. OK Joachim, 75 ohms into a half lambda dipole (impedance of appr. 70 ohms in feeder point) should work fine. Anyway you inspired me for my 4m antenna which is a 2 x 1.04 m (half l.) antenna to be used somewhere next months. Probably with 75 ohms video-coax of which I have a lot. All the best. Ron PA2RF

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  8. Ron,

    that's the spirit... hahaha!
    Yep, fingers crossed for 70MHz being a ham band in Holland soon. The few Watts we will be allowed should not pose any problem to the 75Ohms stuff, I guess. Cheers, Joachim

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  9. Joachim,

    Good Post!

    I have been using RG-6Q for a long time, and love it!

    The price is right, good quality connectors are easily available at the hardware store. I do only recommend slide on connectors that are pushed on via a special hand vice.

    I use RG-6Q for all receive only, and for all QRP work.

    I keep several assorted F adapters and use them when necessary.

    73 - Eldon - WA0UWH at http://WA0UWH.blogspot.com

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  10. Eldon,

    thank you very much! Good to know that I am not the only one...

    73, Joachim

    ReplyDelete