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The problem
with making your own trapped HF antennas is usually getting the
coaxial traps tuned to frequency. Some try to use a GDO (grid dip
oscillator, or its solid state equivalent) but the typical dial
frequency calibration on one of these is usually poor anyway. The
other problem is that the actual GDO frequency will usually "pull"
a bit away from the dial calibration depending on the level of inductive
coupling as it goes close to resonance so, again,that method is
useful but is neither quick nor accurate.
Most
people try to do it this way - they use the GDO to find the "dip"
and rely on the accuracy of the GDO dial. Move the coil turns then
dip again & so on to get it onto frequency, maybe using the
station receiver to confirm frequency. The GDO is inherently unstable
(it is an unshielded free-running oscillator after all...) so even
trying to find it on a receiver can be an issue. This process is
very slow and prone to errors depending on how close the GDO coil
is to the trap - that is if you can even see the dip on the GDO.
By the way,
a good starting-point calculator for coax traps for multiband wire
antennas is the Coaxial
Trap Design software by Tony Field VE6YP - { download:
coaxtrap.zip & read the HELP ! }
My technique
is to use my Marconi RF signal generator at +10dBm output into the
coaxial trap (wired in series) and thence into a RF probe and then
into a DMM (an analogue meter is preferred though). All wires to
be as short as possible.
I
set the sig gen to the wanted frequency ( eg 28.300, 21.200, 14.200,
7.080...) and start expanding or compressing the trap turns coarsely
on the former by hand until I see the DMM voltage (2V or 4V range)
start to drop. I then use an insulated alignment tool to fine-tune
the turns and when I am close then I start winding insulation tape
around the first 1/3rd of the winding, re-check, next 1/3rd, recheck
and fine tune again and finally the last bit bar one turn. This
last turn is carefully adjusted for the bottom of the voltage dip
before the last of the insulation tape is wound on.
You
can then check the actual frequency by moving the sig gen frequency
up or down slightly (eg in 10KHz steps) to find the actual trap
resonant frequency, as indicated by the absolute minimum voltage
reading on the indicator meter. It is a good practice to write the
actual frequency onto the trap former with a permanent marker so
you know exactly which band it is resonant on.
By using
the above technique, I can get the traps for each side of the dipole/V
quickly tuned within 100KHz of each other without any problems.
One thing I must mention is that my coax traps ALWAYS have extra
coax (up to 5-10% above the calculated value) in their construction
and are thus always low in frequency and this technique works because
spreading the turns raises the frequency. My trap formers are usually
slotted towards one end to make it easy to spread the turns. Of
course you can always cut a short length off the coax to increase
frequency but it is usually a major rewind if it is too short to
begin with.
You can also
use this same technique to check the tuning of the traps from multi-band
yagis although the connection arrangements have to be done with
care.
These
formers are ready to wind some new traps for a multiband inverted-V
for HF field day operations.
The trap pairs
left to right are 7, 14, 21 and 28 MHz.
See
also my pages about field
day antennas and the W8010 modifications
using about coax traps.
If you don't have a suitable sig gen then use the HF transceiver
set to the wanted frequency on it's lowest transmitter RF output
power setting ( < 1 watt) into a 50 ohm dummy load via a T-piece
and then use a piece of coax with a 470ohm to 1Kohm 1-2W series
carbon resistor into the trap input and use the same adjustment
procedure as above. Don't use an inductive style series resistor,
and by the way, this series resistor is actually part of an attenuator
network to keep the RF power applied to the RF probe low - DON'T
OMIT IT.
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While
searching for something else recently, I came across a web page
( http://www.marcspages.co.uk/tech/2104.htm
) that uses the trap assembly ( conventional LC or coax styles )
in the coupling path between two induction loops. The same arrangement
was found elsewhere at http://www.qsl.net/dk7zb/Trap/trap.htm
and that one was accompanied by a series of photos that provide
good insight into the construction of the coupling loops & the
physical testing arrangements.
(Marc's image)
The
RF source is a suitable transmitter at a low RF power level (typically
1-2 watts) into one side. The pickup loop is then fed into some
form of detector - an RF probe, suitable high frequency oscilloscope..
In many ways, the RF probe with an analogue meter is the best option
as it makes the signal peak a lot easier to discern.
Just
a suggestion, if you already have a good 50 ohm dummy load, make
up a cable from the SWR meter that puts the coupling loop in the
centre conductor circuit (instead of the 50 ohm load) and then take
the other side of the loop to a coax connector and screw the 50
ohm load onto that. Doing so puts the one side of the RF load at
'ground', and the coupling loop at full RF - but since the coupling
is 'loose' and the RF power is low, no real safety issues should
occur.
It
is possible to just use the SWR meter to tune the traps but
don't forget to include the in-line 50 ohm resistor ( of a suitable
construction for HF and power rating ) - as shown above. The trap's
resonant frequency is where the SWR is highest - typically as found
by changing the transceiver's frequency. Alternatively, the procedure
could be as I suggested further above - set the transmitter frequency
and then adjust the trap frequency by spreading or compressing the
turns so that the SWR peaks there.
Finally,
you can even omit the SWR meter and just use the pickup-loop detector
method. Trap resonance coincides with maximum signal at the detector
in this arrangement.
Or,
as Marc suggests, there is no reason that you cannot use both indication
methods similtaneously.
The
only thing about this coupling methodology is that it can't be used
for tubing antenna traps ( eg from trap-style yagis - tribanders...)
- you can't really inductively couple to them - but my method using
direct connections further up the page works with ANY style of antenna
trap !
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