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Radio antenna reception issues

Thomas Phy

I'm New Here
Has anyone had problems with transmitting and receiving based on where the tower was located to you while sitting on the ground? I've ran into two occasions where the tower could not hear me and I couldn't not hear them. As I re-positioned, I could then hear but not transmit. In the air, I don't seem to have the same issues but, sometimes it seems that I need to be with-in 5 miles to hear AWAS.
RV-12, Garmin radio, antenna on the bottom. frustrated.
 
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A few times on the ground, once in the air (very nose low, pointed right at the tower, 15 miles out) I have had communications problems with a tower using my belly antenna. Always solved by either (1) turning 30 deg or so, or (2) switching to #2 com and wingtip antenna (both 1 and 2 work).
AWOS signals are deliberately low power, but 5 miles seems very limited. Check that the antenna and the coax are properly grounded ( and sitting on a ground plane).
 
Try replacing the coax connector on the "antenna" end of the coax. The center pin crimp is sometimes iffy. This routine fixed two airplanes.
 
Similar issues

I operate out of a controlled field and have similar issues communicating with the tower. Where I taxi out I nearly point directly at the tower and communication is usually good, but not always.
After run-up I need to reposition pointing towards the tower to hear them clearly and vise-versa. If I'm sideways, wing blocking antenna, reception is likely poor. Pointing at the tower I have better results.
Now in the air transmissions work well.
 
Belly or top mounted makes little difference.

While VHF can be thought of as "line of sight" it can and does take multiple paths between transmitter and receiver.

If you are having these problems I'd first look to an install issue, not an antenna location issue.

Carl
 
Belly or top mounted makes little difference.

While VHF can be thought of as "line of sight" it can and does take multiple paths between transmitter and receiver.

If you are having these problems I'd first look to an install issue, not an antenna location issue.

Carl

I disagree... a typical bent VHF belly mounted antenna can be affected by a close by ground plane. How many inches from the horizontal portion of your belly antenna to the ramp surface? Also remember that the antenna should be vertically polarized. :)

I have proved this the hard way at Tucson airport - see previous posting, and yes, I had clear line of sight to the tower from the West ramp. Lots of concrete and rebar there, I think it may have been the old ANG ramp.
 
I too have gotten from the Tower, "Transmission garbled and unreadable", while on the ground at certain times, only to have them confirm, "loud and clear" once I moved. My latest radio issue is a background hum both I and the receiving radio can hear.
 
? Also remember that the antenna should be vertically polarized. :)
mp.

Bent whips are, for the most part, vertically polarized. Most of the radiated energy comes from the vertical part. The last half is necessary for impedance matching but contributes only a minor part of the radiated energy.
 
Bent whips are, for the most part, vertically polarized. Most of the radiated energy comes from the vertical part. The last half is necessary for impedance matching but contributes only a minor part of the radiated energy.

Not much of the semi-standard belly mount Comant antenna is even close to vertical.

ART.03100.jpg


Only 8.75 inches (of the approx. 24 inch 1/4 wave distance) is at an almost 45 degree angle. :)
 
Not much of the semi-standard belly mount Comant antenna is even close to vertical.

ART.03100.jpg


Only 8.75 inches (of the approx. 24 inch 1/4 wave distance) is at an almost 45 degree angle. :)

The way quarter wavelength antennas work is current is 90 degrees out of phase with voltage. Current is at its maximum at the base, or feed point. Voltage is close to zero at this same point. At the end of the antenna, current is close to zero and voltage is at its maximum.

Power is radiated where there is current - at the base. The end of the antenna is needed to keep the overall system at resonance, and with the appropriate feed point impedance.

So - bottom line a bent whip antenna will act (for all practical measurements) just like a standard vertical antenna.

Carl
 
Definitely double check all center pin connections on your coax. I fly frequently at a towered airport and have never had an issue in almost 230 hours of flying. I don't think the placement is an issue or many more -12 owners would be having a problem.
 
The way quarter wavelength antennas work is current is 90 degrees out of phase with voltage. Current is at its maximum at the base, or feed point. Voltage is close to zero at this same point. At the end of the antenna, current is close to zero and voltage is at its maximum.

Power is radiated where there is current - at the base. The end of the antenna is needed to keep the overall system at resonance, and with the appropriate feed point impedance.

So - bottom line a bent whip antenna will act (for all practical measurements) just like a standard vertical antenna.

Carl

Yes, but how does it work if there is a honking big ground plane - all of the rebar in the concrete ramp - less than an 1/8 wave length away and parallel to the horizontal bit of the antenna? :)

Do you think that was a tested mode?
 
Thomas, you didn't say if this was a factory or amateur built airplane, and how old it is. Workmanship and age can make a difference. Regardless, the first place to look for a problem with the symptoms you describe would be the antenna system. Antenna itself, coax feed line, connectors, grounding etc. take it from an old "ham" (K2GOC). If it was me I would dissassemble all above and test and clean all the components. It only takes a little oxidation or corrosion to make a bad connection.
 
On a side note does the com antenna need to be mounted with stainless steel hardware? The std nuts kept loosing so i replaced them with steel nyloc nuts. Is there any issues with that? Thanks for your time
Chuck
 
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