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G8MNY > TECH 30.12.09 09:19l 112 Lines 4581 Bytes #999 (0) @ WW
BID : 45957_GB7CIP
Read: GUEST
Subj: 160m 7 Ferrite Rod Aerial
Path: IZ3LSV<IK2XDE<DB0RES<ON0BEL<F4BWT<GB7COW<GB7CIP
Sent: 091230/0021Z @:GB7CIP.#32.GBR.EU #:45957 [Caterham] $:45957_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New May 08)
(8 Bit ASCII graphics use code page 437 or 850)
This is a design used in a local club construction project to make a small 160m
aerial that can be used on low power Tx.
To get the signal level up on Rx & Tx it uses 7 large ferrite rods to form a
large bundle.
()() These are covered with a heat shrink tube or tape to form the tight
()()() tight strong bundle. Two windings are then wound on them, the high
()() Q resonant winding is spaced out, & the 4 turn coupling winding is
put on in between them.
THE COILS
The resonant winding covers most of the ferrite with a spaced out thick well
insulated wire.
ÚÄÄÄÄ/Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/////////Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/¿
ÃÄÄÄ/ / / / / / / / / / ///////// / / / / / / / / /Ä´
³ / / / / / / / / / / ///////// / / / / / / / / / ³
ÃÄ/ / / / / / / / / / ///////// / / / / / / / / /ÄÄÄ´
À³Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/////////Ä/Ä/Ä/Ä/Ä/Ä/Ä/Ä/ijÄÄÄÄÙ
³ ³COUPLE³ ³
Many Turns for 4 Turns
Resonant Winding
CIRCUIT
_____
||( ³
COUPLE ||( ³_
Rig o)ÄÄÄÄÄÄÄÄ¿ ||( ³/|
50ê ³ _ )||( === C2
³ ³³/| )||( /³TUNING
ÀÄÄ´ÃÄÄÄÄÙ ||( L ³
/³³ ||( ³
C1 ||(_____³
RESONANT
CAP VALUES
C1 is 150-250pF & can be made up with fixed 150pF 500V capacitor & a variable
100pF in parallel.
C2 is a large air spaced capacitor that has to handle the power you want to put
in. It has to resonate the L you have wound & have a large insulated knob to
keep it balanced when your hand is on it tuning. Values of 2x 500pF @ 500V for
an L of 10 turns, down to 30pF @ 7kV for 40 Turns are suitable for about 30W.
In theory the peak voltage could be as high is this..
___________
Peak Voltage = Q * 1.4 * û Watts * Xc ( = Q * 223V for 30W)
Where Xc = 834ê for a C2 in mesh of 100 pF @ 1.9MHz.
& Q = the gain, less the wanted radiation loss.
The Q can be estimated be the Rx bandwidth/F for a -3dB drop or Tx SWR 5.9
Loss dB VSWR
0 Â .úÄú. Ã 1:1
1 ´ .~ ~. Ã 2.7:1
2 ´ / \ Ã 4.1:1 With 30kHz Bandwidth
3 ´ /<---3dB--->\ Ã 5.9:1 Q = 1900/30 = 63
³ Þ Bandwidth Ý ³
6 ´ Ý (30kHz) Þ Ã 14:1
³ Þ Ý ³
³ Ý Þ ³
10 ´ / \ Ã 40:1
³_..ú-Ä~ ~Ä-ú.._³
ÀÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÙ
1.85 1.9 1.95 MHz
BANDWIDTH
As with magnetic loop aerials the Rx is well protected from most of the band
noise & strong out of band signals due to the high Q, & with poor or broadband
front end Rx this can be a great advantage over all the signals a long wire
present to the Rx.
ARCING
At resonance on Tx there is very high voltage across the tuning capacitor & it
is liable to arc over if there are any imperfections like dirt or bent close
parts of the plates (corners). Dielectric caps (other than mica) are generally
not suitable for Tx, as the losses cause heating & melting of the dielectric.
On test in FM once it starts arcing you get a Jacobs ladder effect & the arc
will be maintained following an airborne dust strike (turn off), but this is
not a problem for SSB & the same peak RF generally will not cause an arc.
SATURATION
The ferrite may start to saturate (in the middle) after about 10 Watts, but C1
can be adjusted to improve the match at the higher power. Heating is only very
slight for a 30W carrier after several mins. (do not touch it with Tx RF as U
may get a deep RF burn!)
STRONG RF FIELDS
If used for Tx the magnetic flux of the end of the rods is EVERY HIGH & can
easily damage some electronics if close, so keep ICs & /M phones away!
IN USE
It is quite directional & very selective, with a sensitive Rx (preamp on), the
normal band noise floor can be heard. A single QRM source can often be nulled
out enabling quite good Rx.
But the gain on Tx of course will be 30dB or so down compared to a proper
matched long wire aerial or dipole, so only locals can contacted with it, &
being so directional limits the usefulness on ham nets I have found.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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