ARRL

Secure Site Login

Forum Home - Rules - Help - Login - Forgot Password
Members can access, post and reply to the forums below. Before you do, please first read the RULES.

feedline in pvc

Dec 12th 2016, 20:49

K7WXW

Joined: Oct 28th 2016, 11:33
Total Topics: 0
Total Posts: 0
I am planning an 80m dipole that I want to feed with 450 ohm ladder line. The feedline will run from the antenna to an attachment point on the house and down to a grounded entrance panel. I am concerned that the RF voltage on the feedline, at least where there is some possibility of people or animals coming in contact with it.

If I place the ladder line inside a PVC conduit from the house connection point to the entrance panel, will I change the impedance or other characteristics of the feedline? Are there other reasons not to do this that I haven't thought about?

Dec 12th 2016, 23:32

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

Ladder line is low loss because it is surrounded by mostly air. Replacing the air with lossy materials will increase the loss. Perhaps the most problematic material is water--wet insulators will increase the loss. PVC is an unknown in that it contains fillers that may or may not increase the loss. In theory, you could measured the return loss or SWR when the line is shorted or open, but you may run into measurement issues when measuring very high SWRs.

You can get capacitive coupling through insulators at RF. And, the high voltages that result when running an amplifier can burn through the PVC insulation of common house wire used as antenna wire.

Zack Lau W1VT
ARRL Senior Lab Engineer
Dec 13th 2016, 16:31

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The commonly available ladder line is fully insulated -- like the old TV "twinlead" except with large perforations ("windows"). It is safe from direct contact, but you would not want livestock or other critters to get too close if you're running high power. (There would be heating from the intense RF field.)

There are significant RF fields for some distance away. Rule of thumb: keep it separated from conducting or dielectric objects by about 10 times the lead spacing, to minimize coupling (mismatches or imbalances) and losses. Short runs through narrower tubing (such as through a wall) should be OK.

You can evaluate insulator loss by (very carefully!) testing in a microwave oven. If it heats noticeably after a few seconds, you have a problem. Vapors from overheated plastics can be dangerous, so don't overdo it. (And there could be domestic jurisdictional issues, too. :)

73 Martin AA6E
Dec 16th 2016, 00:17

K7WXW

Joined: Oct 28th 2016, 11:33
Total Topics: 0
Total Posts: 0
Thanks for the guidance. One more question... why do I read in QST and other publications that RF burns are a danger with exposed ladder line?
Dec 16th 2016, 02:17

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
"Traditional" ladder line (often called open wire line) was made from uninsulated copper wires together with ceramic or other insulators spaced every foot or so. RF burns would be an issue. If you use the fully insulated modern type, the hazard is less, but you still wouldn't want to handle wire carrying much power. You could get zapped from capacity coupling with the wire. Keep in mind that ladder line is often used in high-SWR situations, where the voltage maxima can be quite high (kilovolts) even for moderate power. Treat it with respect!

Back to Top