In message <3.0.1.32.19970609155155.0069482c@sirius.com>, Charles Parker
<silas@sirius.com> writes
>Could you please give a detailed discription of how to make such an antenna?
Probably if you have the vertical tail height the simplest form is an
transformer end fed halfwave or a J antenna as it better known which
would be overall around 63" (1.6m) tall. (assumes 12.5mm ribbon)
Nasty Ascii time
<--------------1060mm---------------><------522mm----->
||
Top || 15mm int
<--> approx 53mm
Firstly the drawing needs rotating 90 degrees as it is a vertical
aerial.
Buy enough tape to do some initial experiments and use a substrate as
close to the aircraft's plastic skin as possible, a couple of layups of
scrap bid on cling film over a flat surface.
Stick two lengths of tape one 1600mm long and the other 540mm long with
a 15-18mm parallel gap between them. Run a short length of tape across
the two ends and trim off the open ends to approximate the above
dimensions. The dimensions are all internal. Solder the tape joints
together to form a continous connection.
Attach a piece of 50 ohm coax to the points marked X and tack solder
them in place. Inner to the short leg, outer braid to the long leg and
run the coax downwards across the bridge before turning it away.
This is the part you need to go find a radio ham chum who has a field
strength meter for exactitude. Mount the antenna above ground at the
lower end and quirt a bit of RF up the coax with the radio tuned on
127mhz or therabout and adjust the height position of the coax
connections at X for best received level. The alternative is to insert a
VSWR meter in the coax with a 765mm long piece of coax between the X
connections and the meter and adjust the X locations for lowest return
loss (greatest forward power). The X connections need to be kept
parallel. If when tuned it comes out lower than 127Mhz trim a fraction
off the two legs in the ratio 2:1
ie. 2mm off the long leg and 1mm off the short leg. 127Mhz is
approximiately mid voice band, 126.685 to be precise.
VSWR across the band 118-136 will probably be about 1.5-1.6, wider tape
would help. With the signal budgets for AM radio it's not worth worrying
about the precise detail.
The indicated position won't be far off as is and for most purposes will
work. Problem is without knowing the dialectric constant of the fin's
structure being precise isn't practical.
Once your dummy antenna works then it is easy to translate it to the
plane. The upper 1060mm part is the radiating element and needs to be as
near perpendicular to the flight line as can be arranged. The lower U
section is an impedance transformer and is notionaly balanced so it
doesn't radiate, This part can be formed around the curvy bit of the
tail cone and may even be tilted slightly without detrimental effect but
should be kept away from any vertical metal bits. The upper leg will be
parallel with the rudder hinge and it is better that it is close to
rather than positioned away from else the hinge will start acting as a
parasitic element and skew the radiation pattern away from it's normal
doughnut shape.
The points marked "o" are RF "hot" and there shouldn't be anything
metallic near them else the antenna will detune. The last 100-150mm of
the upper leg can be tilted over slightly if space is at a premium
without too detrimental an effect. The bottom U can be rotated almost 90
degrees but the radiation pattern will become skewed and the matching
will go out the window..
Run the coax across the middle of the base and then forward to the
cabin. Gobs of bid and goo to hold it in place and a couple of small
patches over the terminals to secure them against vibration.
If you contact Jim Weir at RST he has the where-with-all to make a half
wave dipole which is 2/3rds the height of the J and provides the same
gain performance. His needs a few extras like odd lengths of twinax
cable for a transformer and a few ferrite beads but he can supply all
the parts and the construction method.
Both methods provide a featherweight DC grounded antenna which should
help reduce the pops and crackles from the ignition.
Next week how to make a slot antenna for the nav end of the band.....
--
Graham E Laucht
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