Some hints/tips on how to assemble low-loss WIRE TRAPS !


Wire trap for 12m WARC band

Before we start to assemble our traps, here some general info as introduction :

Example : Wire trap for WARC band inverted-V (12 & 17m band)

  
  To construct a wire trap, you will need a suitable capacitor. According to some sources, the value in pF should be chosen around 1.5x the band for which it is intended (so for example, for 20m band trap = 30 pF, 80m = 120 pF).

The best choice is a 'doorknob' type because these are real transmitting capacitors designed for high voltage. You regularly find these on ham flea markets, I purchased a couple on Ebay in Ukra´ne.  Watch out with Russian doorknob capacitors: there are 2 types : K15U-1 and KVI-3. The first type is best suited for this type of application. The model I had available is 68 pF/6 kV.  I checked these capacitors with a magnet : they are completely a-magnetic, including all the screws and washers (this is a must as they will be located inside the coil).

Off course you need as well wire. I took 5mm dia aluminum wire (Hint: next time you buy flowers for your XYL, ask  your florist, he can get it in rolls of 20m length, they use it for decoration).

For the 12 / 17m inverted-V, a trap resonating on the 12m is required, so let us say on 25.2 MHz.


(Click picture to enlarge)

 

Now first calculate the value of the inductor (L) to build a LC circuit with C=68 pF. You can use a handy online calulator, for example

http://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml      

This gives us about 0.6 mH as result.

Now we need to design a coil with this value. I traced a smart online calculator here:

http://www.darc.de/f39/bastel/2003/berichte/Helical Coil Calculator.htm

You can 'play' with dimensions till you have approximately the required inductance, but off course the number of windings must be an entire value (for mechanical construction) The 'best coils' are those with 'square' appearance (diameter about same as length) and wire spacing at least equal to wire diameter... I found that a value of 4 turns with coil diameter of 52,5 mm and 7mm spacing was suitable.

 

  The coil is wound and formed on a suitable pipe (I used silicone sealant cartridge, dia. is approx  50mm) The ends of the coil are fitted with eye lugs. Now it is time to mount the capacitor : mine was fitting nicely inside a 32mm PVC tube (thick walled, 3mm, resistant to heat version). It must be soldered with 4 eyelugs to both M5 stainless steel bolts.

The calculator shows to be very correct! The exact resonant frequency (use a dip meter) can be fine tuned by stretching or compressing the coil. Finally, glue a cap on the tube side which will be upwards so that no rain can enter the tube. The lower side can remain open. (Inverted-V)

During my tests, the antenna works fine with 100w power. Above this level, the capacitor seems to warm up - probably by the high RF current by the very high Q-factor. This causes the capacitance value to drift, the circuit gets out of resonance and the SWR increases quickly > 3.  So, this design is not useable for high power....