The AM Forum

I am building an antenna tuner using some parts with scrote and thought it would be fun to mount it on a nice piece of wood and encase it with plexiglass. It seems to me I have seen this done by other hams.
A friend of mine expressed concerned about radiation and possible harmonics.
My feeling are
1.That the harmonics should be suppressed before they leave the transmitter
2. Any radiation is of little concern since TVI etc is no longer a concern.
I welcome all thoughts on the matter.
Carl
WA1KPD
 

I tend to agree with both of your statements Carl. I ran my "big-boy" tuner completely uncovered for decades and never ran into any problems other than now and then a spider would take up residence between the plates of one of the big variable caps. Needless to say it would vaporize with a very satisfying "zzzzzooooorrrrchhh" when the big rig was keyed.

:O)

Hi Carl,

Yes, the harmonics SHOULD be handled by the transmitter. If they are down -40dB or so, the FCC will most likely not hear you. Depending on the antenna, feedline and freq, there can be another 10-40 dB of ANTENNA harmonic suppression available. (on the harmonic freqs using a coax ant)

Generally, the more complex the tuner, the more harmonic suppression. A simple parallel floating "JJ" style tuner with one cap and coil may be good for only -25dB additional suppression. The more complex split capacitor designs with capacitors to ground may give us -40dB or more at the harmonic freqs.  If more suppression is not needed, then why bother?

I have always used the simple floating design because the harmonic suppression of my rigs is good - a pi-network usually sees to that.

In your case it should be easy to measure your stock second and third harmonic suppression using a local RX or SDR receiver (TX into a dummy load) to see if you even have a problem to begin with.

Yes, TVI is not an issue anymore unless the neighbor has rusted cable connectors or some issue that causes diode rectification or poor shielding. In that case, most any rig, good or bad harmonic suppression, will tear them up.

But bottom line, do everything "textbook" and it will all add in the proper direction to give cumulative good results.  Measurements are key.

T

Hi Tom,
Was using your design anyway, sans torrid and a bit more C since I have it.
2 questions:
1.Where do you get the thicker pieces of Plexiglass you use
2.How do you cut, drill it?
Thanks
Carl

Hi Carl,

I bought my Plexi, usually 1/8" or 3/16" thk,  at a local plastics company. Check the web for plastics suppliers. They will cut to any size you need.

For cutting I use a band saw.  A table saw will work too, with a fine tooth blade, but be REAL careful. The Plexi will often launch itself using a table saw. Read up on precautions.

Plexi drills very nicely with a standard drill press. I also use hole saws to cut bigger holes. Again, be careful when the bigger drill bits grab and spins the material. Always use heavy leather gloves and try to tie down the piece.

T

Thanks Tom!

Your local Lowes and Home Depot should have sheets.  Look for the Lexan brand or type, it is easier to cut with saws and drill with standard drills.  

Never had a problem with breadboard construction.  Here is a picture.

Best regards,
Fox

Fox-
Those are CAPACITORS......
Jim-
Thanks I will check them out!