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Old 13-04-2011, 12:37   #1
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Coaxial Cables Question

I recently bought an ICOM AIS receiver, which is hooked up between the antenna and my VHF. The really great thing about this is that ICOM don't supply the short coaxial cable from AIS to VHF, haven't they heard about plug and play.

So, all I was left to do is find a connecting cable, which was unsuccessful, so I went down to the local electronic shop and bought myself two PL-259 plugs and some coaxial cable, before soldering all this together I decided to check the coaxial cable on the internet and to my astonishment I found it was 75 ohm impedance, "I asked distinctly for 50 ohm".

Can anyone tell me if I can use the 75 ohm cable for this short connecting cable, i.e. about than 15cm/6" long. And what effect will this have if any on broadcasting and reception.

Would very much appreciate some help with this!

Steve
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Old 13-04-2011, 12:45   #2
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Re: Coaxial cables question

Make the lead up and try it, you have little to loose
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Old 13-04-2011, 13:01   #3
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Re: Coaxial cables question

Reception won't be affected but you could get some reflection because of the impedence mismatch in the transmission. The only way to know for sure is to check it with an SWR meter. If you could find a short length of RG8 or RG58 it would be better.
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Old 13-04-2011, 13:02   #4
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Re: Coaxial cables question

Steve,

The Icom AIS receiver has about a 3.5db insertion loss on receive (that's about 33%). The VHF transceiver is designed for 50-ohm coax, which the existing VHF antenna cable is likely to be. No sense further reducing VHF sensitivity by inserting a mismatched cable, IMHO.

You should have no trouble finding a 50-ohm short jumper coax cable...RG-8X would be preferable.

Bill
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Old 13-04-2011, 13:16   #5
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Re: Coaxial cables question

For the price of a short length of the correct coax, I'd change it, but you won't actually lose much by using the 75 Ohm coax (about 1dB of TX power). The radio may back off on the power output due to the reflections.
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Old 13-04-2011, 14:45   #6
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Re: Coaxial Cables Question

Thanks guys for your help!

Cut off 10" from my emergency antenna, soldered on the plugs, did a continuity test and "Bob's yer uncle", problem solved.

RG58 was the key!

Must get another PL259 for the emergency though!

Steve
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Old 13-04-2011, 15:03   #7
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Re: Coaxial Cables Question

Steve,

Glad it worked out.

Get 2 or 3 PL-259's...good to have on hand. Be sure to get right reducers...they're different for RG-58 and RG-8X.

Bill
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Old 17-06-2011, 08:46   #8
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Another coaxial cable question.

I am in the process of installing an Icom M710 and an Icom AT130 tuner.

Can I use RG58c/u coax between the tuner and the radio?

I have read that the RG58 is only suitable for 100 watts. The M710 is rated at 150 watts output I believe.

I am using an insulated backstay antenna and the tuner will be approx 3 meters from the radio.

Steve
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Old 17-06-2011, 09:40   #9
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Re: Coaxial Cables Question

150 watts is not a problem, RG58 can handle a thousand watts at HF frequencies into a matched load.

Eric
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Old 17-06-2011, 09:55   #10
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Re: Coaxial Cables Question

If you want to use a lower loss and more robust cable use RG8u. Better for a longer run, but bigger diameter.
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Old 17-06-2011, 11:07   #11
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Re: Coaxial Cables Question

Steve,

If you must use the thin coax, then use RG-8X. Lower loss, better for this application.

However, if you can manage to fit RG-213 or, better, RG-214 (double-shielded) coax, then it might be good to do so, and this tends to attenuate RFI from outside sources as well as from the radio to other instruments aboard.

Any length will work fine.

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Old 17-06-2011, 11:12   #12
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Re: Coaxial Cables Question

He's using less than 10' of coax. It's not going to matter at these frequencies.

Eric
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Old 17-06-2011, 11:13   #13
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Re: Coaxial Cables Question

Attenuation...yes, it won't matter.

RFI...maybe, and worth the trouble IMHO.

Happy Weekend,

Bill
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Old 17-06-2011, 18:36   #14
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Thanks guys.
I'll give the rg58 a go and see how it works. I wasn't sure if it would handle 150 watts.
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