Making a multi-band antenna using traps
A few tips on using traps
The introduction of our easy-to-tune antenna traps has encouraged a whole host of people to try making their own multi-band antennas. That's great as making antennas is fun and can be very satisfying. Most folks start with a co-ax fed dipole - which is generally a single band antenna. It's an easy antenna to make and is a great performer. Adding bands using traps takes the fun to a new level, plus you will learn a little about tuned circuits too.
Let's dive straight in and make ourselves a multi-bander. We will make a trapped dipole for 20 m and 40 m. The first stage will be to make a 20 m dipole. The RSGB has a useful page of tips for that right here. The RSGB suggest that a balun is not really necessary in most portable situations, I agree. However my experience suggests that a balun is essential with a trapped dipole as getting the adjustments right is very hard if the feeder is too closely coupled into the antenna system. Note that we make the dipole for the highest frequency band first. In this case 20 m is 14 MHz and 40 m is 7 MHz so we make a 20 m dipole.
[Handy hint] If you are intending to use your multi-bander as an inverted vee antenna, make sure that you use the same apex angle for your 20m dipole that you will have when the complete antenna is made, the apex angle of an inverted vee will affect how it works. There is a useful page on making an inverted vee here. The author uses single strand wire, I would use multi-stranded wire for a portable antenna for easier handling.
Once your 20 m dipole is working you can prepare for the next stage. We want our traps to act like end insulators for our 20 m dipole so that adding the 40 m extensions does not affect its resonant frequency. Traps are a parallel tuned circuit - a capacitor in parallel with an inductor. This type of circuit has a very high impedance (i.e. looks like an insulator) at its resonant frequency; therefore we want a resonant frequency of about 14 MHz. It's a good idea to have the resonant frequency of your trap a free percent BELOW the band as this reduced any potential trap losses. Thus we will adjust our 20 m traps for resonance at about 13.7 MHz. The exact frequency is not critical but do try to make both traps resonant on about the same frequency (say +/- 25kHz).
Once you have your traps adjusted, they can be attached to the ends of your 20m dipole: now the fun begins. You need to add some additional wire onto the traps to extend your antenna to get it to work on 40 m - but how much wire do you need? It's easy to work it out with a calculator. Just work out the length of each leg of your 20 m dipole (about 5 m) and then work out the length of a 40 m dipole (about 10 m of wire) and take one away from the other... 10 - 5= 5 m. So you need to add another 5 m of wire to traps for 40 m.
Once you have added the 40 m extensions. put the antenna up again and check that it still works on 20 m. It should work on 20 m just as before. If not, something is wrong. You must sort out the 20 m problem before doing anything else. If 20 m is working as before check out 40 m. If the traps are working properly you will notice that your antenna is resonant below 40 m. Why? Tuned circuits look like an inductor (coil) below their resonant frequency so your traps look like little loading coils on 40 m. That's handy as it's easy to shorten the 40 m sections by cutting a little wire off the ends. Do this until your antenna is resonant where you want it on 40 m. Because of the loading effect of the traps, the usable bandwidth of the 40 m section of your antenna will be a little less than that for a full-sized 40 m antenna.
[Pro-tip] By changing the L-C ratio of your traps you can trade off bandwidth vs antenna length. Note that the L-C ratio of co-ax traps is not adjustable.
Making your new two-bander into a three-bander is also possible by adding additional traps and extensions. In this case, you can only easily add a band lower in frequency than 7 MHz so you could add 80 m (3.5 MHz). The bandwidth on this third band will be even less. My experience is that three bands is about the maximum for a trapped dipole system. Beyond that the adjustments get increasingly tricky!