Cb radio, discone, 75 ohm cable

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Cb radio, discone, 75 ohm cable

Postby Southwales » Fri Jan 26, 2018 1:22 pm

Hi, Being quite rural I never thought I would find anyone using cb FM, over the last 24 hours my uniden scanner has picked up 2 occupied channels on my recent home discone antenna, I could not pick these up on any other antenna from my home, as it is the discone that harps achieved this how is this antenna likely to be at transmitting as it is fitted with 75ohm cable, I thought I have read in the past on here that 75 ohm cable is not much good for tx, problem is If I change to say rg58 the signal loss could mean I don't receive that well.

Also I plan to build a 2nd discone so one for my scanner and one for the cb, the cb one having larger elements, my attic is only small as it is a converted dorma bungalow, the 2 home made antennas would only have about 3 ft gap between them, how would this affect them?, would only use one antenna\device at a time. The coax would only need to be 12ft on this one so again would I get away with pf100 75ohm coax?
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Re: Cb radio, discone, 75 ohm cable

Postby G4RMT » Fri Jan 26, 2018 7:38 pm

75 Ohm cable will be bad for transmitting - the VSWR will be very high. Transmitting is a totally different thing. A discone for 11m will be a terribly big beast - over 8ft tall, and the top disk will be tricky too! As you have a decent loft space - then you could build a ΒΌ wave, with some radials, or string up a slanting wire half wave dipole, although the polarisation won't be perfect.

Keep in mind the antenna you build is NOT a discone - it's a bi cone - which would need to be nearly 5m tall to give a decent match to 50Ohm cable. It's receiving some CB in it's too short form, because it's a bit of metal up in the air, not because it's actually much use at those frequencies.

50 Ohm for transmit, but cheap cable for 11m isn't too lossy - but the size will be difficult. A discone or bi-cone for hairband and above won't match well to even 50Ohm cable.
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Re: Cb radio, discone, 75 ohm cable

Postby m0lsx » Fri Jan 26, 2018 9:06 pm

Radio waves are a sine wave, or a Lazy S as the old American Cattle men called it.
Image

Poor SWR (more correctly VSWR.) Happens when that sine wave does not fit properly onto the antenna & part of the power that the radio produces is then sent back down the coax, as there is in effect a miss match between radio wave length & antenna wavelength. So the length of the antenna is important.
The calculation is speed of a radio wave, which is 300,000 meters per second, divided by Kilohertz. This gives you the wavelength of the frequency. That is, it gives you the length across that Sine Wave.

Image

The impedance of the signal at the radio needs to be 50 Ohms or close to & using 75 Ohm coax would cause you issues as someone who does not yet understand the basics.
When setting up a antenna for transmitting you need to check it's SWR & get it as low as you can, to avoid damaging the radio. And that SWR needs to be done in position. You can do an approximation else where. But when in position the SWR may well be very different, due to any number of factors.
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Re: Cb radio, discone, 75 ohm cable

Postby Southwales » Sat Jan 27, 2018 11:50 am

Hi, Thank you for the advice, have you got any images of an a easy to make cb antenna that I can make preferably using my 2.5mm earth cable left over from my bi-cone build, and what coax do you recommend for a run of about 12ft?

Going back to my bi-cone that I made for my scanner with 8 vertical elements, I am thinking of making another slightly bigger version with maybe 12 vertical elements this time, would this help pull in more signal or can you have to many elements?
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