VOR antenna question

[Mike M]

My plane was purchased with a cat-whisker type VOR antenna mounted atop the vertical fin. Stainless elements, looks like a Comant CI-157 but it's from the 70's and I can't find the receipt to tell make/model. I removed a KX145 and its rack and didn't install a nav radio. (It worked when removed, except the frequency selector readout failed and couldn't reliably tell what freq it was on.) Ran the nav antenna cable to a bulkhead connector on the panel so I could connect a jumper to my handheld Narco HT870. Tried it in flight, no reception. Rubber ducky works, but with this connection, nothing.

Queried internet for ideas. Best I could find, one element goes to center pin, one goes to ground. Ground is also the cable shield. Continuity test? OK, try it. Clip to port element, check to center pin on end of jumper, continuity. Check to shield, also continuity. Hmm. Clip to starboard element, check to center pin on end of jumper, continuity. Check to shield, ditto. Double hmm. Check jumper, nope, no continuity between center pin and ground. Check bulkhead connector, yep, continuity between center pin and ground. Triple hmmm.

I'm confused. My limited knowledge says no way can both the center pin and the shield go to ground and still deliver a signal to a radio. The fin tip is fiberglassed around the antenna and will have to be drilled/ground off to gain access to the connections on that end. Is there something else I should check before doing surgery, or is it really supposed to work that way?

[WLIU]

Your understanding is correct. There should be no continuity between the center conductor and the shield or ground. Somewhere there is a connection. To make the antenna work you have to track that down and repair it.

But with all of the availability of GPS devices, in aviation specific boxes and on tablets that have a built in GPS and talented apps, why not take the antenna off the airplane? Doing so will pick up a little speed and make the airplane look better. And you will navigate more accurately and with better situational awareness.

Best of luck,

Wes

[skyfixer8]

I have run into this problem before where someone has changed the BNC connector either at the antenna, radio end or someplace in between. If you decide to keep the system as is. look for a piece of the ground braid (looks like a hair) contacting the center wire on the cable inside the connector. It is pretty easy to take it apart.

[FlyingRon]

Your understanding is correct. There should be no continuity between the center conductor and the shield or ground. Somewhere there is a connection. To make the antenna work you have to track that down and repair it.
Actually, that's not necessarily true. The meter is DC, and there may indeed be continuity between the two. A dipole is a balanced antenna and the coax feedline is not. THere's probably a balun of some sort built into the antenna that depending on design may be a DC Short.

[Mike M]

I pulled bunchastuff to get reasonable access to run of antenna cable from panel to base of vertical fin. Found a cable junction under panel about 1.5 ft from radio rack. Disconnected, checked continuity going each way from connection. No continuity between center pin and shield going aft, but continuity between pin&shield going toward panel. Pulled bunchamorestuff to remove that section of cable, found hidden crushed spot. Replaced that section of cable. Reinstalled allapulledstuff. Works fine now.

Roger the advice about pulling VOR antenna. Reason I don't - no IFR GPS. Total package right now is 1 Comm, GPSMAP 196, Android tablet and smartphone each with Avare, KT76, and Narco 870 for VOR approaches. Considering Sporty's SP400 to get ILS capability. VOR and ILS approaches without a radar or DME requirement are getting rarer each day, but are the cheapest emergency IFR procedure I know about.

Thanks for the advice, folks. Found the problem!

[Eric Brown]

You also may want to consider the radio range when using the horizontal VOR antenna as a com antenna. I was told, at one time, that the VOR signals are transmitted horizontally and the com signals are transmitted vertically. Thus the difference in the orientation of the two types of antennas. Using a com unit on a VOR antenna will limit the com range.

[Mike M]

You also may want to consider the radio range when using the horizontal VOR antenna as a com antenna

The KX 145 required separate nav and com antennas, so the nav antenna wasn't used to transmit with that one. From the Narco HT870 manual: "...it is not recommended that HT870 COM transmissions be made while using a VOR antenna."

Good enough for me. By the way, I had misplaced my HT870 manual and looked all over the internet for one before really doing a serious physical look. The roaches and silverfish have been at it a bit, but it was good enough to scan. Anybody want a copy? I tried to email Bob Nucholls since he has so many manuals on his site but couldn't find his email address.

[deej]

Bob Nuckolls is nuckolls.bob -at- aeroelectric.com

[Mike M]

Thanks, deej...sent Bob a copy.