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G8MNY > TECH 28.11.18 12:24l 98 Lines 3380 Bytes #999 (0) @ WW
BID : 47070_GB7CIP
Read: GUEST
Subj: Understanding Trapped Antennas
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Feb 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
BASIC TRAP
Here is a simple 40m trap used in each side of an 80m dipole.
1/2 of the Dipole or a 1/4 wave wire
<---------- Electrical 1/4 wave on 80m e.g. 17.7m ---------->
<------1/4 wave on 40m-----> <------- e.g. 7.8m --------->
__________________________()))))____________________________
³ 1/4 wave at 40m ³ ³
³ e.g. 9.9m ÀÄÄ´ÃÄÄÙ
Feeder 7.1MHz Tuned
Circuit
Note the resonant lengths are slightly shorter due to the added coil wire.
HOW IT WORKS
The L & C have to make a high impedance Z at 40m to isolate the hot end
(high Z) of the 40m dipole.
Z = Q * Xc
This means the L must be quite large & C small to get the tuned circuit Q
multiplied by the capacitance reactance Xc (= Xl at resonance) to make the Z
very high.
The Voltage & Current rating is quite high, e.g. 2kV AC for 100W.
MULTIPLE BANDS
It is possible to add a series traps 1 per band, isolating the resonant aerial
length from the rest.
1/2 of the Dipole or a 1/4 wave.
<------------------------------ 1/4 wave on 80m -------------->
<-------- 1/4 wave on 40m -------->
__________________()))))_________()))))_______________________
³ 1/4 wave at 20m ³ ³ ³ ³
³ ÀÄÄ´ÃÄÄÙ ÀÄÄ´ÃÄÄÙ
Feeder 14MHz Tuned 7MHz Tuned
Circuit Circuit
SOLID AERIALS
With solid aerials & beams the traps for say 10m & 15m are often combined. The
outer sleeve forming both capacitors & also becoming the middle conductor, it
is connected to a tapping on the coil.
1/2 Dipole or 1/4 wave vertical.
10m L&C 15m L&C
__________________
________________________ ³ __________________
________________________())))))__________________³
Ally Tubing ________³_________ Thin Tubing
Wide Tubing
<-- 1/4 wave @ 10m -->
<--------- 1/4 wave @ 15m ---------->
<------- Electrical 1/4 wave @ 20m -------------->
With this design water is a major problem, & drain holes are needed on the
underside of the wide tubing.
Each length can sometimes be adjusted, so that the low SWR sweet spot can be
se p per band, starting with the highest frequency.
POWER HANDLING
At resonance the trap voltage & current can be very high depending on the
power. e.g. @ 50W on half of a dipole @ say a Z of 10kê, means the trap has to
isolate a voltage of :-
___________
V = û50 * 10,000 = 707V RMS or ñ 1kV peak
At 400W into say a vertical rod aerial the trap voltage could be:-
_____________
V = û400 * 10,000 = 2kV or ñ 2.8kV peak
As the traps are not used (resonant) for the lowest band, generally they will
not be power limited. e.g. a limit of 100W on 10m & 15m, but 400W on 20m is OK
on a 10/15/20m tribander.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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