APRS cowl antenna

Several threads back I mentioned that one of Ryan Howell's J-pole antennae was ready to install in the cowl of N399SB. I've been using a 1/4 wave whip on the belly of the plane and it has provided superb APRS coverage. But since my rig was recently upgraded with a Micro-Trak 8000 it seemed timely to see if a concealed antenna would provide similar performance to that of the external unit with a 300mw transmitter.

Pete Howell gets excellent results with the J-pole in the wingtip of his RV-9A but I'm not aware of anyone who has installed the antenna in the engine cowl. My initial thoughts were the antenna probably wouldn't perform too well but the experiment was simple to conduct and....maybe it would exceed expectations.



Installation was certainly easy, the antenna was merely glued to the lower cowl with some contact cement. It was possible to get about half of the antenna in a more or less vertical orientation. I located the antenna in the forward portion of the cowl to get it away from the exhaust headers and as far away from the engine mass as possible (but still a safe distance from the flywheel!)

A BNC-BNC bulkhead connector was installed in the firewall so the antenna could be quickly disconnected when the cowl was removed for service. A short BNC-SMA cable connected the MT-8000 that is located behind the instrument panel.



I flew the antenna for a couple flights before actually hooking it to the transmitter to make sure it wouldn't be damaged by heat in the engine compartment and to confirm the glue would stay, uh, glued. It survived just fine, so after making a local flight with the whip, the cowl antenna was connected and a flight in the local area with known APRS coverage undertaken for comparison purposes.

More flights would be necessary for a comprehensive test, but based on the initial flight it was obvious the cowl antenna was inferior to the whip. This shouldn't come as any surprise since the external 1/4 wave whip is most likely the optimum antenna for our application. Likewise, this is no reflection on the quality of the J-pole antenna because we are asking it to operate in an environment for which it was never intended. The APRS trace of the flight appeared on aprs.fi but there were several missing beacons, and overall performance was less than the original Micro-Trak 300 with the external antenna.

However, if an external antenna was out of the question for an APRS rig, and for some reason a wingtip or canopy installation was unfeasible, the cowl antenna may be an option. It did work with my APRS rig, just not nearly as efficiently as the external antenna. I suspect its performance with the Micro-Trak 300 would indeed be marginal if located in the cowl.

Well....that is what experiments are for.....now we have data we didn't have before. I may take the J-pole and install it in a length of PVC pipe on a mast for use here at the office with the APRS iGate I'm considering rigging up. That is much more in keeping with the original design intent of a J-pole and it should make a fine iGate antenna.

Bummer!

Sam,

I'm sorry it did not work! I continue to have pretty good luck with the j-pole in the wingtip, but agree that the whip will be superior in our application for those that want to mount an external antenna.

I wonder if having all the metal and moving parts in the engine compartment might have affected results? The firewall and fuse would block most signal to the rear. I think we get the same effect in the wingtip from the end of the wing to the stations on the far side of the plane.

There go my plans to retire on J-pole antennas.

I am going to try a re-tuned J-pole as a backup comm antenna just to see how it works.

petehowell said:

Sam,

I'm sorry it did not work! I continue to have pretty good luck with the j-pole in the wingtip, but agree that the whip will be superior in our application for those that want to mount an external antenna.

I wonder if having all the metal and moving parts in the engine compartment might have affected results? The firewall and fuse would block most signal to the rear. I think we get the same effect in the wingtip from the end of the wing to the stations on the far side of the plane.

Click to expand...

Not a problem Pete, I had pretty low expectations for the reasons you stated. The cowl antenna would probably get enough packets onto the aprs server to leave a usable breadcrumb trail but some of us have gotten spoiled to having nearly all the beacons make it to the map. My test flight was <1500AGL, at higher altitude it may spit out quite a few beacons. The antenna is still in the plane, I'll give it another shot soon.

Here are the two flights:



The first flight with the whip was the one that went west to Courtland. I don't think I lost a single inflight packet and even got some on the ground at 9A4 and DCU (that Micro-Trak 8000 is a beast!). The "cowl" flight was the short, northern loop and several packets got dropped, even though the general outline of the flight was recorded.

The cowl antenna works, just not as well as a whip, and probably not as well as your wingtip installation.

By the way, the hits from the ground at DCU were within 3' of field elevation.