I have searched the net and this forum, and am surprised no one has really discussed it. It is something I have been doing for quite a while for receive and TX antennas, and with good results, so I thought I would post my general experience doing so.
This is a general overview of my experience using cat 5, and cat 3 wire for antenna feed lines.
I have found that it works pretty good as long as you take it’s properties into consideration.
General things to keep in mind.
It has a 100 ohm nominal impedance.
It has an attenuation curve a little bit higher than run of the mill rg58. That means it is useable for longer runs 14mhz and down, while only usable for short runs at 14Mhz on up.
The velocity of the 4 pairs are pretty much the same. It is a result of the stringent timing requirements of 100Mbps+ Ethernet. So it works good as a set of four matched transmission lines in one jacket.
I have generally found that a single pair can comfortably handle about 100W SSB from 14Mhz on down. (24awg) If you try to run 100W 2M FM into it, the first 20 or so feet of line will get very hot very quickly.
Approximate attenuation for normal off the shelf cable.
150Mhz 7db per 100f
100Mhz 6db per 100f
50Mhz 4db per 100f
25Mhz 3db per 100f
15Mhz 2db per 100f
Below 15Mhz is the roll off point where losses drop to basic IR2 levels.
General things I have found from real world use.
It is a balanced feed line so it works best for feeding balanced center feed antennas. It is poorly suited for feeding end feed antennas. Just as 50 or 75 ohm coax is poorly suited for driving center feed antennas.
A properly tuned half wave will show a feed point impedance of 75 or so ohms, and 75 ohm RG6 and RG11 is the standard rule of thumb for driving such antennas. But you can still use a 100 ohm line without a matching transformer with acceptable mismatch.1.25-1.5 to 1 SWR. It generally produces better results because the feed line is inherently balanced which will reduced the feed point complications that you generally get with feeding the antenna with RG6 RG58 or RG58.
With a wider band antennas the feed point impedance general goes upward which generally makes it a better match to 100 ohm line, than 50 or 75 ohm line.
Older tube type transmitters with a load control are will generally be able to nominally load into a 100 ohm feed line.
My HW101 and Kenwood TS520 handle it just fine with loading control adjustment to spare.
Newer solid state rigs should use a matching transformer to make their finals more happy. Otherwise you will see about a 2 to 1 SWR.
For general reception only use on the SW/AM bands. It works great. I have never had a problem forgoing a matching transformer at the radio. Most receivers never present a true 50ohm to the feed line anyway, so trying to match the feed line to a 50 ohm load that doesn’t exist is an exercise in futility. A simple ferrite balun is all you need to reject common mode signals.
100 ohm line generally works better than 50 ohm line for wideband receive only antennas. In that regard 300 ohm line is better than 50, 75, or 100 ohm line.
A single cat5 cable makes a great multi antenna feed line for a set of half wave dipoles orientated in different directions.
You can also use cat3, or untwisted POTS telephone wire, but it will pick up a lot more noise from stuff inside the house that it runs close to on it’s way to your receiver. And RF attenuation will be a lot higher in the VHF frequency range. The cat5 cable provides a lot better RF noise immunity than cat3. I have never really checked if it’s better than RG58, but I would say they are pretty close.
It makes an outstanding AM antenna feedline.
And above all….. KEEP THE WATER OUT OF THE JACKET.
Water on the outside does not appear to bother it much, but when it get water inside the jacket, it’s higher frequency attenuation figures take a massive hit. If you get water in it, you will be able to tell pretty dang quickly.
In general, when deciding what feedline to pick for an antenna project, it does a nice job of filling the massive hole between 300ohm twinlead and 75 ohm coax. And it works good for times you want a lower impedance balanced line.
I have found that It opens up a whole new area of design possibilities, and being able to use RJ jacks will quickly spoil you.
This is a general overview of my experience using cat 5, and cat 3 wire for antenna feed lines.
I have found that it works pretty good as long as you take it’s properties into consideration.
General things to keep in mind.
It has a 100 ohm nominal impedance.
It has an attenuation curve a little bit higher than run of the mill rg58. That means it is useable for longer runs 14mhz and down, while only usable for short runs at 14Mhz on up.
The velocity of the 4 pairs are pretty much the same. It is a result of the stringent timing requirements of 100Mbps+ Ethernet. So it works good as a set of four matched transmission lines in one jacket.
I have generally found that a single pair can comfortably handle about 100W SSB from 14Mhz on down. (24awg) If you try to run 100W 2M FM into it, the first 20 or so feet of line will get very hot very quickly.
Approximate attenuation for normal off the shelf cable.
150Mhz 7db per 100f
100Mhz 6db per 100f
50Mhz 4db per 100f
25Mhz 3db per 100f
15Mhz 2db per 100f
Below 15Mhz is the roll off point where losses drop to basic IR2 levels.
General things I have found from real world use.
It is a balanced feed line so it works best for feeding balanced center feed antennas. It is poorly suited for feeding end feed antennas. Just as 50 or 75 ohm coax is poorly suited for driving center feed antennas.
A properly tuned half wave will show a feed point impedance of 75 or so ohms, and 75 ohm RG6 and RG11 is the standard rule of thumb for driving such antennas. But you can still use a 100 ohm line without a matching transformer with acceptable mismatch.1.25-1.5 to 1 SWR. It generally produces better results because the feed line is inherently balanced which will reduced the feed point complications that you generally get with feeding the antenna with RG6 RG58 or RG58.
With a wider band antennas the feed point impedance general goes upward which generally makes it a better match to 100 ohm line, than 50 or 75 ohm line.
Older tube type transmitters with a load control are will generally be able to nominally load into a 100 ohm feed line.
My HW101 and Kenwood TS520 handle it just fine with loading control adjustment to spare.
Newer solid state rigs should use a matching transformer to make their finals more happy. Otherwise you will see about a 2 to 1 SWR.
For general reception only use on the SW/AM bands. It works great. I have never had a problem forgoing a matching transformer at the radio. Most receivers never present a true 50ohm to the feed line anyway, so trying to match the feed line to a 50 ohm load that doesn’t exist is an exercise in futility. A simple ferrite balun is all you need to reject common mode signals.
100 ohm line generally works better than 50 ohm line for wideband receive only antennas. In that regard 300 ohm line is better than 50, 75, or 100 ohm line.
A single cat5 cable makes a great multi antenna feed line for a set of half wave dipoles orientated in different directions.
You can also use cat3, or untwisted POTS telephone wire, but it will pick up a lot more noise from stuff inside the house that it runs close to on it’s way to your receiver. And RF attenuation will be a lot higher in the VHF frequency range. The cat5 cable provides a lot better RF noise immunity than cat3. I have never really checked if it’s better than RG58, but I would say they are pretty close.
It makes an outstanding AM antenna feedline.
And above all….. KEEP THE WATER OUT OF THE JACKET.
Water on the outside does not appear to bother it much, but when it get water inside the jacket, it’s higher frequency attenuation figures take a massive hit. If you get water in it, you will be able to tell pretty dang quickly.
In general, when deciding what feedline to pick for an antenna project, it does a nice job of filling the massive hole between 300ohm twinlead and 75 ohm coax. And it works good for times you want a lower impedance balanced line.
I have found that It opens up a whole new area of design possibilities, and being able to use RJ jacks will quickly spoil you.
