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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 07-01-2016, 10:32 PM
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Lightbulb Using cat5 UTP for antenna feed line.

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.
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Old 07-03-2016, 3:07 PM
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For grins and giggles I pulled a new untouched 1000F spool of cat5e riser cable off the storage shelf and did a few test on the work bench.

Using a TDR unit with a variable termination load, the nominal impedance was right around 100 ohms as expected. No cable irregularities were detected.

Conditions for attenuation test was an injection voltage of 14.14Volts peak.
Or 10V RMS.
With a nominal impedance of 100ohm.
That means drive current is 100mA RMS.
Total injection power is 1W RMS.

Total attenuation figures for a single 1000 foot pair of a cat5e cable is as such.

100KHz 2.5db
500KHz 3db
1MHz 4.4db
1.5MHz 5.2db
2MHz 6db
4MHz 9.1db
8MHz 11db
15MHz 14.9db
21Mhz 18db
30MHz 22db
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Old 07-03-2016, 6:11 PM
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Quote:
Originally Posted by Hatchett View Post
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.
Interesting test, thanks for posting this.

Twist rates are different for each of the 4 pairs to reduce cross talk. There would also be a very slight length difference between the pairs.

Would be interesting to see this test done using shielded CAT6.

Also, you can get outdoor rated CAT5 & 6 cable filled with the "iky-pic" water displacement gel. Would be good for outdoor use without having to worry about water ingress.

Nice thing is this cable is so cheap.
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Old 07-03-2016, 8:36 PM
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Quote:
Originally Posted by mmckenna View Post
Twist rates are different for each of the 4 pairs to reduce cross talk. There would also be a very slight length difference between the pairs.
Hm…….
I just did a quick check and the largest variation in pulse arrival time I measured between pairs is about 20nS for the 1000F spool.

That equates to a phase reversal for a 25MHz signal over 1000F
Or a 90 degree phase shift for a 12.5MHz signal over 1000F

For a 100F run, those figures would be 250MHz, and 125MHz respectively.

Average variation between pulse arrival time for the next closest (in arrival time) pair was about 5 to 6 nS.

That equates to a phase reversal for a 100MHz signal over 1000F
Or a 90 degree phase shift for a 50MHz signal over 1000F

For a 100F run, those figures would be 1000MHz, and 500MHz respectively.

For lower frequencies the shift would be negligible for a normal feed line run. For higher frequencies (VHF) it would be something to keep in mind. Pair selection to obtain delay matching could be done.

Or you could trim length at termination end to obtain an exact length match.
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Old 07-03-2016, 10:59 PM
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I have done this with an Icom Marine VHF/HF radio and have had a video posted online for a while now. This was to prove that with an antenna tuner, Cat5 could be used for a field expedient antenna.

Not the best antenna, but if done right, will work. Thank you for sharing the details of what you have used.
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Old 07-06-2016, 8:21 AM
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Interesting work by Hatchett. I wouldn't think that this cable can handle 100 watts all that gracefully, but I guess if the losses are low, it can't be all that bad. His work reminds me of those old Zip-Cord dipole designs.

Allow me to point out one thing I believe he forgot to mention: Make sure the wire pairs you use are the ones that are twisted together! So use the blue-white, white-blue pair, not blue-white orange-white. I used to know a guy who was color blind. He would attach RJ-45 connectors to cables and the continuity would be okay --but you couldn't get any data down that cable because he didn't match up the pairs properly.

Another issue: most Cat 5 cable is NOT rated for sunlight and NOT rated for moisture. It will degrade faster than most coaxes will. That said, for temporary use it may not be too bad. It sure is cheap.and readily available.
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Old 07-06-2016, 3:57 PM
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Originally Posted by ab3a View Post
I wouldn't think that this cable can handle 100 watts all that gracefully, but I guess if the losses are low, it can't be all that bad.
Looking at it in a technical point of view…

Cat 5 specs a max of 125 VDC at 600ma.

That limitation mainly comes from the RJ connectors.
Most Cat 5 cable is actually rated 300V.

NEC specs 24awg wire at 2A RMS.

So, technically speaking, If you go low enough in frequency, one pair should be good for about 400W RMS. That is if you forgo the use of the RJ jacks.

When you start going up in frequency then thermal limitations start becoming a factor.

I would honestly say you could get away with 400W continuous wave below 2Mhz. Once you get up into the 14mhz and higher range, then thermal limiting is a concern with 100 CW.

Just the same as it is with RG58 which has about the same loss characteristics.

If you try to operate 500W CW into RG 58 at 2MHz, You are in good shape.
If you try to operate 500W CW into RG 58 at 400Mhz, it will melt quite quickly.
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Old 07-06-2016, 4:03 PM
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Also, it's not uncommon for equipment to be powered over CAT 5-6 cable. Power Over Ethernet is becoming more and more common with VoIP phones, IP cameras, etc. Running a few watts DC at 48 volts down the cable is not an issue. Of course, as stated above, RF is a different animal.
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Old 01-31-2018, 9:34 AM
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Default Cat6

I came across this thread wondering if I could cheat on getting an FM radio antenna outside a metal building using cat5 cable that I had laying around rather than buying coax. This information is excellent. Huge thanks to Hatchett for posting this.

I'm curious. Cat5e has a rating of 100 MHz, while Cat6 has a rating of 250 MHz. Could one assume that Cat6 would have less loss at VHF frequencies than Cat5e cable?
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Old 01-31-2018, 6:53 PM
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The 100 ohm impedance should be for one twisted pair. You might be able to parallel two pairs and have a 50 ohm feedline. You could probably short each pair out and use two shorted pairs for one half of a feedline and the other two shorted pairs for the other half. Not sure what the impedance would be but probably lower than 100 ohms due to more surface area.
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Old 01-31-2018, 9:49 PM
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Buy the co-ax. CAT5 is unshielded and running it through a metal wall will cause havoc with impedance.
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Old 02-01-2018, 10:44 AM
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Quote:
Originally Posted by Ed_Seedhouse View Post
Buy the co-ax. CAT5 is unshielded and running it through a metal wall will cause havoc with impedance.
The Cat6 I'm asking about is shielded.
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