G8MNY TECH 30.08.19 06:15l 89 Lines 3047 Bytes #365 (0) @ WW
BID : 19459_GB7CIP
Subj: Understanding Trapped Antennas
Path: OK0NBR F3KT OK0NAG DB0ERF DB0RES DB0OVN DB0GOS ON0AR GB7CIP
Sent: 190830/0909Z @:GB7CIP.#32.GBR.EURO #:19459 [Caterham Surrey GBR]
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 ---------
__________________________()))))____________________________
│ 1/4 wave at 40m │ │
│ e.g. 9.9m └──┤├──┘
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 --------
__________________()))))_________()))))_______________________
│ 1/4 wave at 20m │ │ │ │
│ └──┤├──┘ └──┤├──┘
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
__________________
________________________ │ __________________ ________________________())))))__________________│
Ally Tubing ________│_________ 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 set up 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 10kΩ, means the trap has to isolate a voltage of :-
___________
V = √50 * 10,000 = 707V RMS or ± 1kV peak

At 400W into say a vertical rod aerial the trap voltage could be:-
_____________
V = √400 * 10,000 = 2kV or ± 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?

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