Question about building a capacitor

[K1JJ]

Hola!

I'm building up an 6M antenna matching network to be mounted on the tower to match 20 ohms j12  to 50 ohms. Tom/KLR designed it in this thread:

http://amfone.net/Amforum/index.php?topic=8606.0

Anyway, the homemade 76 pf capacitor is a 6"X 3" copper plate that is mounted an 1/8" above the aluminum box bottom.  The problem is if I use air for the dielectric, (die =1.0005) the capacitance is down around 30pf or so. If I use a dielectric like a sheet of Plexiglass or fiberglass with a higher dielectric constant, (die = 2 or 3) the capacitance will double or triple, depending on the material and spacing.  This would give me the necessary 76 pf to play with.


I'm wondering  if an 1/8"  plexiglass  or fiberglass sheet dielectric at 50 mhz and 20 ohms is a lossy proposition?

Also, does the dielectric plate actually have to TOUCH the copper plate to give the higher constant or does it simply have to be in the path between the two capacitor plates?

T

[WA1GFZ]

Yes it has to touch or you will have to account for AIR gap.
Check plexiglass I not sure what it does at 50 MHz.
Fiberglass will absorb moisture so it will be unstable unless you can seal it.

I saw a guy use different diameters of copper tubing to make a cap by sliding one inside the other. There was a lead screw on one to make it adjustable.

How about stacking plates to get more C. Air will be the most stable and least dielectric loss.

[W1RKW]

If plexi is lossy you may want to think about heating effects also.

[KB2WIG]

Plate glass maybee??

[W2VW]

Use Teflon.
Works great for low inductance capacitors.

[WU2D]

Hi Tom

Is it a Gamma Match or a Delta?

For my homebrew 6M Gamma Match I used the 1/8" sliding rod inside the tube trick, exactly as shown in the old VHF handbook. Both were aluminum and I made spacers out of three chunks of coax poly dielectric insulation, drilled to fit tightly on the rod. This all aluminum approach held up until I took the beam down - well over 10 years with no issues.

Mike WU2D

[K1JJ]

Hi Mike,

Well actually it's eight, stacked 4el 6M Yagis and they are all matched with delta matches and 4:1 coaxial baluns - and the 8-bay phasing harness is working FB. The problem is I am presented with a final harness impedance of 20 ohms j12 to match to the main 50 ohm hard line.

So, we are building a box with a sheet of copper making a 76 pf capacitor to ground and a small coil to the harness input. This should give a 1:1 match with some tweaking.

I may climb up there with the box and try different plate sizes and dielectric materials until the MFJ -259 likes it... 

Frank: Tnx for the info, OM. Think I will start with Plexi and see how that does...Teflon has a die constant of only 2, whereas plexi is as much as 3.  More plates wud be a hassle at this point.

T

[W2VW]

Teflon can be found in very thin sheets. Plexi dunno. Puncture voltage is not bad either.

[WU2D]

Hi Mike,

Well actually it's eight, stacked 4el 6M Yagis and they are all matched with delta matches and 4:1 coaxial baluns - and the 8-bay phasing harness is working FB. The problem is I am presented with a final harness impedance of 20 ohms j12 to match to the main 50 ohm hard line.


T

Wow Tom - I thought my two element single director beam was cool but beamwidth too tight- Hi

Mike

[KL7OF]

Tom...test a piece of plexi in the microwave oven...If it makes it there, it will make it at 50 mhz....gud luk...Steve

[WA1GFZ]

Tom,
I still have that roll of 3 mil kapton. That would be rated at 21 KV if you need any. I'll be in Enfiled all weekend.
I have used mylar at 1 GHz to tune radar antenna strip line. I wonder if you can still buy it at say art stores. Mylar is rated at 1 KV / mil.
The higher the dielectric constant the more unstable it is.
How about double sided PC board?? Then seal the edges with epoxy. Peal back the copper on one side around the edge so it doesn't flash over.
I bet 1/8 inch board will take a lot of power. I have used FR4 / G10 at over 500 MHz. gfz

[kf6pqt]

Teflon gets EXPENSIVE the thicker it is... A while back I got a 6x6x 3/8" piece to make a carburetor insulator for a 429 Cadillac... I was only able to talk the supplier down a tiny bit from the EIGHTY bucks he quoted me.

But when it came to mill the 4 holes in it, it cut like buttah! 

-J