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G8MNY > TECH 08.12.05 22:50l 68 Lines 2298 Bytes #999 (0) @ WW
BID : 7664_GB7CIP
Read: DL1LCA GUEST OE7FMI
Subj: Understanding Trapped Antennas
Path: DB0FHN<DB0MRW<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 051208/1736Z @:GB7CIP.#32.GBR.EU #:7664 [Caterham] $:7664_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Aug 05)
BASIC TRAP
Here is a simple 40m trap used each end of an 80m dipole.
1/2 of the Dipole or a 1/4 wave wire
<--------------- Electrical 1/4 wave on 80m ----------->
<------1/4 wave on 40M----->
__________________________()))))_______________________
³ 1/4 wave at 40m ³ ³
³ ÀÄÄ´ÃÄÄÙ
Feeder 7MHz Tuned
Circuit
Note the 80m length is slightly shorter due to the added coil wire.
HOW IT WORKS
The L & C have to make a high Z at 40M to isolate the hot end of the 40m
dipole. This means the L must be quiet large & C small. The Voltage & Current
rating is quite high, eg 2KV 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
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.
Each length can sometimes be adjusted, so that the low SWR sweet spot can be
set up per band, starting with the highest frequency.
N.B. As the traps are not used (resonant) for the lowest band, these
generally will not be power limited, eg 100W on 10m & 15m, >400W on 20m OK.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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