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Author Topic: Trapped inv-V, with traps resonance out of band  (Read 1000 times)
EI2HEB
Member

Posts: 54




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« on: January 09, 2013, 03:29:11 AM »

Hi,

I have a question regarding building a trapped inverted V antenna, for 15/40/80 meter.

Now I have the traps, but they are not resonant in any of the bands, they are resonant on 29.6 MHz (trap1), 14.1 MHz (trap2), 5.5 MHz (trap3)
I understand that traps are "lossy" and that they are "most lossy" at resonance frequency.

I was thinking of a configuration, something like this (detailing only 1 leg):

balun -- trap1 -- 15mSection -- trap2 -- 40mSection -- trap3 -- 80mSection

or (omitting trap1):

balun -- 15mSection -- trap2 -- 40mSection -- trap3 -- 80mSection


Question1: would this be possible ?
Question2: how would you tune it: build all together and then tune the sections, or build one section, tune, then hookup the next section ? or even different again?

Any comments, suggestions, tips and tricks welcome!

Thanks,
73 de EI2HEB - Edwin.
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WB6BYU
Member

Posts: 13243




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« Reply #1 on: January 09, 2013, 08:48:23 AM »

To solve the problem - or even to determine if there is a solution, you need to know
the L and C values of each trap, not just the resonant frequency.  From that you can
calculate the effective reactance on each band.

Then you would model an antenna with different wire lengths and these reactances
at each junction to find a combination that works.  When using traps that are not
resonant at the operating frequency, ALL the wire lengths affect EACH band, so
adjustment is not trivial. 

Usually you would use a trap with a resonant frequency between the two
bands of interest, so you would use traps 2 and 3, but none of the wire lengths
are going to be what you might expect.

Generally speaking, an antenna with two traps and 3 wire sections on each side
will be resonant on 3 frequencies, but the problem is getting those to fall in the
desired bands with fixed-value traps.  The closer the traps are to resonance,
the less the impact that the antenna beyond the trap has on the tuning.


And traps really aren't that lossy:  there is some advantage in moving the resonant
frequency a bit below the band, but losses are relatively low for a well-built trap,
and the simplicity of design and tuning will often argue for using them at or near
resonance.  Also, for an inverted vee you want to keep maximum current closer to
the middle of the antenna where the effective height is greater, rather than in the
end sections that are closer to the ground, and that requires that the traps be
closer to resonance.  Otherwise you end up with something more like a 3/2 wave
dipole with significant current in the outer sections.
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EI2HEB
Member

Posts: 54




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« Reply #2 on: January 09, 2013, 09:16:44 AM »

Thanks WB6BYU, for your quick response!

It seems that the traps I have (given to me) are for 10m-FM, 20m and 60m.
So I guess it would be better (and easier to tune) an antenna for 10mFM, 20m, 60m and 80m, a three trap for 4 bands.

I don't have the LC values of the traps, so I cannot calculate, just experiment !

thanks again,
Edwin.
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WB6BYU
Member

Posts: 13243




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« Reply #3 on: January 09, 2013, 09:30:19 AM »

That certainly is the easiest solution.  You can build your own traps for
other bands as desired.

You can also measure the traps in various ways and calculate the L and
C values from that.  For example, if you measure the resonant frequency
of the trap with a dip meter, then add 100pf and take another measurement,
you can calculate the values close enough to get started.

I would generally recommend starting by choosing what bands you want
the antenna to cover, then figuring out how to use the parts available
to do that.  For example, you could add an external capacitor across the
10m trap to move it down to 15m, or you could use the 60m trap to
cover 40m and 80m.  So with a bit of creativity you may be able to cover
15, 20, 40 and 80m using the existing traps.

Of course, if the learning from experimenting is more important than actually
operating with the antenna (a familiar situation!) then you can try all sorts
of things.  In that case I'd suggest starting with dual-band antennas to start
with, as there are fewer variables involved and it is easier to get a usable
result with cut-and-try methods.
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EI2HEB
Member

Posts: 54




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« Reply #4 on: January 09, 2013, 09:50:12 AM »

You can also measure the traps in various ways and calculate the L and
C values from that.  For example, if you measure the resonant frequency
of the trap with a dip meter, then add 100pf and take another measurement,
you can calculate the values close enough to get started.
Thanks, but I am sure it will take some practice measuring; then again; never to old to learn!

I would generally recommend starting by choosing what bands you want
the antenna to cover, then figuring out how to use the parts available
to do that.  For example, you could add an external capacitor across the
10m trap to move it down to 15m, or you could use the 60m trap to
cover 40m and 80m.  So with a bit of creativity you may be able to cover
15, 20, 40 and 80m using the existing traps.
Never thought of that, intersting approach!

Of course, if the learning from experimenting is more important than actually
operating with the antenna (a familiar situation!) then you can try all sorts
of things.  In that case I'd suggest starting with dual-band antennas to start
with, as there are fewer variables involved and it is easier to get a usable
result with cut-and-try methods.
Yes, maybe a dual band 20/40m to try out first... that might be wise!

Thanks again for your feedback!
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WB6BYU
Member

Posts: 13243




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« Reply #5 on: January 09, 2013, 10:02:47 AM »

In that case, the inner wire would be very close to a quarter wave for 20m,
and the outer wire will shorter (so the total length is less than a half wave
on 40m, due to the loading effect of the trap.)  That's probably the easiest
antenna, since the 20m tuning is relatively independent of the 40m wire
length.  Tune the 20m wire first, then the 40m wire.

Then you can put it up and see if you can notice any difference between
that and monoband dipoles for 20m or 40m, as a test of how lossy the
traps really are.
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EI2HEB
Member

Posts: 54




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« Reply #6 on: January 09, 2013, 10:05:24 AM »

In that case, the inner wire would be very close to a quarter wave for 20m,
and the outer wire will shorter (so the total length is less than a half wave
on 40m, due to the loading effect of the trap.)  That's probably the easiest
antenna, since the 20m tuning is relatively independent of the 40m wire
length.  Tune the 20m wire first, then the 40m wire.

Then you can put it up and see if you can notice any difference between
that and monoband dipoles for 20m or 40m, as a test of how lossy the
traps really are.

Sound like a plan! Thanks for all your feedback and tips!!
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N4NYY
Member

Posts: 4758




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« Reply #7 on: January 09, 2013, 10:27:54 AM »

I built a 20-40-80 trap dipole with Unadilla traps and expertise from antenna gurus on this forum. I kind of shied away from building my own traps because it was not an exact science and the quality would not be nearly as good. I did end up having to trim the wires to get change the center frequency on the 40 and 80. I have across the board under 2 SWR for the entire 20. I have not experience that losses that people talk about, but even if there is, I do not think they are significant. I am very happy with that antenna.
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