MULTIBAND TRAP DIPOLE ANTENNA - (40,20,15,10M BANDS)

 * Simple and Low Cost

* Resonant Antenna, no tuning required while changing bands

* Short in length as compared to half-wave Dipole

Requires careful fabrication & tuning of TRAPS

( Only a few inches of RG58 or any other coaxial cable required for fabrication of frequency Traps)

>No capacitors needed<

 In this type of antenna, coaxial traps work as Loading Coils below their Resonant Frequency, thus reducing the overall length of the dipole - less than 22' length on either side for 40,20,15 & 10M bands!

I used RG59, 75 ohms, low loss foam coaxial cable for feeding the dipole antenna and than transforming it to 50 ohms using a small L & C (more about it later in this article). Even without any matching network, results were good for 50 ohms output from the Tx.

(You could also try feeding the dipole directly from RG58U / RG8 etc.)

 Steps for Multiband Trap Dipole construction and tuning:

1. Ensure that you have 44' or more space for hanging the antenna

2. Start fabrication of the antenna by first making all the traps and tuning them

3. Note that the length of wires interconnecting traps may vary depending upon the construction of traps, antenna location etc. It is advisable to cut the wire lengths 6-10 inches longer than indicated in figure 1. and trim them to resonance for each band.

4. Tuning should start from highest frequency band first - like 10M band in this case. Trim the wire lengths to resonate the antenna at, say, 28.2Mhz, or as per your personal preferances (the Trap should also be of the same frequency)

5. Continue tuning 15, 20 & 40M bands in sequence.

 Fig.1

Fig. 2   Above figure shows final one side of multi-band trap dipole

Basic connections of frequency trap

Fig. 3

Frequency traps can be made from coaxial cable, thus avoiding the use of high voltage stable capacitors. Traps made from coaxial cable have high Q values and have high L/C ratio. Even a small stray capacitance of a few pf is enough to shift the resonant frequency by tens of kilohertz. These can be tuned with the help of Grid Dip meter or Antenna Analyzer used as dip meter.  The Dip Meter coil should not be brought too close to Trap for final adjustment of frequency. This is to avoid large capacitive coupling between the Dip Meter coil and the Trap. Excessive coupling lowers the resonant frequency thus giving a wrong reading on the Dip Meter. 

Fig. 4      Resonant Frequency testing of Trap

Coaxial cable, like RG58U, can be used for making the Traps. Experiment with different lengths of coaxial cable to observe the resonance frequencies. Then incrase or decrease the lengths as per your frequency requirement. You can choose mid band (like 14.15 Mhz for 14.00 to 14.35 Mhz, 20M band) or lower portion of the band, as per your own preference.

You can also fine tune the Trap resonant frequency by soldering a small piece of wire to the centre conductor of the coaxial cable end and trimming the length of the wire to get the desired resonant frequency as shown in Fig. 7. For this, wind the coaxial cable to get, say, 14.80 Mhz and then solder the parasitic coupling capacitor wire  and trim the length to tune the trap to, say, 14.17 Mhz. This works wonderfully well and you can precisely tune the Trap to the desired frequency by avoiding repeated adjustment of coaxial cable length or spacing between the turns.

After resonating the Trap for the desired frequency, give a covering of Teflon tape for weather conditioning. Apply a generous coat of Lacquer followed by paint, Fig. 8 & 9.

Fabrication of frequency traps: 

 Fig. 5 

 Fig. 6

 Fig. 7

Notes

1. There was no difficulty in tuning the Multi-band Trap Dipole Antenna by trimming the wire lengths starting from highest band.

2. SWR varied between 1.4 to 2.0 without any further tuning.

3. By matching the antenna through a L/C impedance matching L network, the SWR could be lowered to less than 1.5 on all the bands. However, this matching network is not essential for all the practical purposes.

 Fig. 8

 Fig. 9

 Fig. 10

 Fig. 11