RG59 coax?

[kiser sosay]

Im curious as to what the effect's would be using a single run for 18' of 75 ohm coax over 50 ohm coax in a mobile install.

I run one 4' wilson superflex roof mounted on cab of silverado. 18' of RG58 coax Mated to a uniden pc76.

I know 75 ohm is usaually for co-phased "trucker style" antennas.
and 50 ohm for single antennas.
This to match the radio.

But I had a cb shop (i won't mention their name) try and sell me RG59 coax for my single antenna.

what effect would this have?

 

[jazzsinger]

An 18 ft run of rg59 could be used to match an antenna with a bad swr/high impedance (i.e, 2:1 = 100 ohms) to a 50 ohm radio.18 ft of rg59 coax at 27 mhz is a 3/4 wave impedance inverter.

in much the same way its used to step the impedance up to 100 ohms in each of twin mobile antennas so that when they are paralleled they give 50 ohms,the main difference is instead of being a step up transformer like in dual mobiles it would be a step down transformer with a single badly matched antenna.

good practice would be to use a 12 ft multiple (impedance repeater at 27 mhz) of rg58 on a properly matched antenna.

it would be one way of misleading someone into thinking their antenna had a better impedance match than it really has.

 

[Captain Kilowatt]

An 18 ft run of rg59 could be used to match an antenna with a bad swr/high impedance (i.e, 2:1 = 100 ohms) to a 50 ohm radio.18 ft of rg59 coax at 27 mhz is a 3/4 wave impedance inverter.

in much the same way its used to step the impedance up to 100 ohms in each of twin mobile antennas so that when they are paralleled they give 50 ohms,the main difference is instead of being a step up transformer like in dual mobiles it would be a step down transformer with a single badly matched antenna.

good practice would be to use a 12 ft multiple (impedance repeater at 27 mhz) of rg58 on a properly matched antenna.

it would be one way of misleading someone into thinking their antenna had a better impedance match than it really has.

Exactly. This is a case where seeing a good SWR at the radio does NOT mean you have a good working antenna. SWR is not everything it's cracked up to be.

 

[HiDef]

There's nothing wrong with using 75 ohm cable as an impedance transformer to match the impedance of your antenna to the rig. I'd just make sure the guy has it figured out correctly. If he does he's no dummy.

IIRC some Wilson antennas come with a length of 75 ohm cable for just this purpose.

 

[kiser sosay]

Thanks for the replies.

I was just wanting to upgrade my coax. To something alittle better than the bottom of the barrel stuff I have in there now.

Now I made a run of rg6 coax (75 ohm) I had extra from running my sat. dish in the house.
The only difference I noticed was my swr was lower by far. Witch was exactly what u guys said would happen. I didnt do any radio checks to see how I was transmitting.

Will I lose performance running that 75 ohm, or is it just tricking the swr meter?

 

[W5LZ]

That could go either direction. If the radio sees an impedance it likes it will tend to put full power into it. Depending on how much reactance is in that 'matching line', there may be more power getting to the antenna. If the radio doesn't like the impedance it sees, they tend to put out less than full power (some of them) so that less is getting to the antenna. It just depends on which of the two instances means more power to the antenna, and that's going to be a guess at best.
The best bet is to make everything in the antenna system the same impedance. And, since the typical radio has an 50 ohm impedance, why not make the whole thing the same? If the whole system is the same impedance then the most power gets to the antenna and is radiated. If it's handled correctly, meaning whatever the impedance happens to be for any particular part of that antenna system, things work out just dandy. That means that impedance transformation for different impedances has to be done, and in the most efficient manner possible.
On the average, using all 50 ohm stuff is just easier than mixing it with some other impedance. If you can handle the required transformations, great. If not, stick with the 50 ohm stuff.
- 'Doc

 

[HiDef]

A good mobile antenna might not ever exhibit a 50 ohm impedance. It depends on the installation loaction and a bunch of other factors. If you match it with a specific length of 75 ohm cable and read an accurate low VSWR at the radio everything is OK. You cannot fool an SWR meter. Your cable is performing the same function as many matching systems whose's use never gets questioned.

The only drawbacks are you have an extra hunk of coax hanging around and the peak voltage and current in the feedline may be higher than a flat run of 50 ohm cable terminated in a 50 ohm load. Unless you are using very light 75 ohm cable and/or running some industrial power it isn't a problem.

You will deliver the exact same power to the load with that matching cable (provided it gives the desired match) except for some very tiny losses which will not make any difference whatsoever.