Tuning Multi-Band Dipole
Building a 10-15-20 "catwisker" type dipole. What element sould I tune first??? I don't want this to turn into an exercise in Frustration !!! As we all know this can turn into fast. Thanks to all that reply...
You always start from the inner elements and work out, whether it's a trap dipole or trap vertical. (Highest frequency band and work down). Generally this goes very quickly.
"A minute of measurement trumps a decade of debate."
Originally Posted by N4MTB
Another name for these type of antennas is "Fan dipole" or parallel dipole. I've never heard catwisker but thats very discriptive.
It is best to tune the LOWEST band first, or LONGEST WAVELENGTH. This is only logical if you think about it. The longer the wavelength, the less precise you have to be with antenna length. Let me explain.
Lets take 80 meters for example. One half wave on 80 meters is approximately 132 feet. A basic dipole is a 1/2 wavelength antenna. 1/2 wave on 75 meters is approximately 123 feet. This means **the difference** between THE ends of the 75/80 phone and CW band is about 9 feet of wire.
I'm not using math with engineering level accuracy here, but stay with me.
On 20 meters a 1/2 wavelength is approximately 33 feet. Now, what we call the "20 meter" band is actually nestled between 20 and 21 meters. THE ENTIRE BANDWIDTH of that band is only a portion of ONE METERS WORTH of bandwidth, whereas the 75/80 meter band has 5 meters worth of bandwidth.
The difference between the ends of the band on 20 meters would ONLY BE ABOUT THREE FEET of wire.
As the frequency goes up, the less wire that it takes to make a significant change in an antenna.
If we got down to the 50 Mhz band, we might be talking about less than ONE FOOT worth of wire to seriously change the resonant point on an antenna.
THEREFORE, to answer your question with logic and simple math, it is best to tune your lowest frequency, longest wavelength elements first and work your way upn in frequency to tune the next. This is because the shorter elements are more difficult to tune and the effects of coupling to the other wires are more pronounced. Therefore, cutting a couple of inches off of your 10 meter wires will have very little effect on the 15 and 20 meter elements, whereas cutting the 20 meter element, if done LAST, might have a serious effect on the 10 meter and possibly 15 meter elements.
Tune lowest band first.
My favorite mode? Morse, of course.
Eric, to avoid confusion, I do not believe he is referring to a trap dipole...By "cat whisker" I am thinking he means a fan dipole (if my visual of what cat whiskers look like is correct).
Originally Posted by KL7AJ
If this is what he means, then I believe the very opposite advice would be true and he should start out with his low band wires first.
My favorite mode? Morse, of course.
Sorry for the poor terminology , but I am refering to a fan dipole. Specifically for 10-15-20 meter phone.
I've always trimmed the higher-frequency elements first, then worked out. Always a "tuned once, and worked". I've never had to double-back on other elements. I've only built like 3 of them, so YMMV.
I've built a number of them and always tuner the lower frequency (longer) elements first. In my experience, the lower frequency elements affect the tuning of the higher frequency elements more than vice versa.
Yes, trim the lowest frequency dipole first; then move up in frequency etc.
The potentially larger dimensional changes on the longer dipoles have more effect on the shorter dipoles than vice-versa; so you want to do the longer dipoles first. You can still expect a few iterations.
Eric's advice is fine for trapped antennas, but not for a fan dipole.
Yes, trim the lower frequency pair first.
Also, the wider the elements are separated, the less interaction.
I'm in the longest to shortest camp myself, but I found that with 6" wire spacing I needed several trips up and down with the last tri-bander [also a 10/15/20 meter]. But once tuned, they are great antennas. I love the coaxial cable savings!