BNC Connection Help

[mlw450802]

Originally Posted by az_gila

The spacer is probably not a good idea. The antenna is sized for the correct frequency at 1/4 wavelength (about 3 inches in this case) and needs a metallic ground plane to work correctly.

The spacer has increased the effective length of the antenna by a measurable percentage.

Yes.... I know "it works", but it doesn't work as efficiently as originally designed, or as required in any TSO (I believe the transponder system is one that does have to meet TSO specs. even in our homebuilts)

gil in Tucson

I thought the radiating length of the antenna was measured after the coax termination point not the distance from the tip to the ground plane. Can you enlighten us here with a little more antenna theory and perhaps dispell a little more of the witchcraft surrounding these things?

inquiring minds would like to know...

-mike

[szicree]

Comant actually recommends a spacer (saddle) in the case of a non-flat belly skin.

Steve Zicree

[thallock]

Gil,

The spacer does not increase the length of the antenna, at all. I assume that the spacer is grounded to the skin, and is part of the gound plane. You can see that there is an insulator (rubber, I assume) between the spacer and the antenna. The spacer only raises the antenna above the ground plane, and by an extremely small amount. Looks like a good fix to me.

Tracy.

[mark manda]

here's a pict of my antennae after removing the spacer.

[az_gila]

It's a quarter wave above the ground plane, not the end of the co-ax.

The original picture was of a transponder antenna, right?

A 1/8 spacer at comm. wavelengths (around 23 inches) is negligible... at transponder wavelengths, it's no longer negligible.

The first section of this describes 1/4 wave antennae, the rest gets into 'black magic'

http://www.web-ee.com/primers/files/antenna.pdf

gil A

[szicree]

The spacer is metal and therefore becomes part of the ground plane.

Steve Zicree

[SteinAir]

For the outrageous amount $17.50 you can save yourself the hassle of fuddling around with spacers, lord knows how much time fabricating the thing, or using ring terminals, et.al. The spacer idea is fine and shows the ingenuity of builders, but once again reminds me of a band aid. Buy the right angle BNC connector and be done with it. No flames intended, and it's just my 2 cents as usual.

Cheers,
Stein.

[szicree]

I'm with you Stein, but I'm not really sure the right angle deal will get me the clearance I need. The 90 degree adapter that's on there now looks about the same size as the photos I've seen of 90 degree connectors. I guess I'm not really sure how much clearance I need. I've currently got around 0.5 - 0.75 inches, which is probably plenty. It just bothers me to see that thing sticking up in such a potentially serious spot.

By the way, do you happen to sell the connector pins that go in the back of my TDR 950 tray?

Steve Zicree

[az_gila]

Originally Posted by szicree

The spacer is metal and therefore becomes part of the ground plane.

Steve Zicree

Steve,

No, not at RF frequencies. ... because the spacer is not a quarter wavelength in radius.

Make a spacer that is a disk and a diameter twice the length of the transponder antenna, and then this statement would be true.
The spacer would then become the ground plane.
RF waves are measured to surfaces at this frequency, not just equal electrical potential....

gil A

[Mike F]

I have the connector and pins for the TDR-950. Take a look at

http://stores.ebay.com/Ferrer-Aviation-Services

Mike F
RV-7QB