DCA Build
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Four Dipoles, comprised of 20mm diameter heavy duty conduit with inspection T's.
15mm x 0.7mm copper tubing with hand fettled PVC internal spacers.
Two half wave dipoles have 36cm RG58 coax with two ferrite beads on each dipole.
The other two dipoles have 72cm RG58 coax, again with two ferrite beads each.
Ferrite beads came from old USB cables.
Dipoles are set to a four way conduit box and a 30 degree tilt to the left, with a spacing of one quarter wave.
5mm clear plastic sheet is formed for support plate.
Spacers are added for further support and to compensate for inverted four way conduit box.
The coax termination for testing purposes, red tape to indicate center conductor terminal.
Three inch drainage pipe with four 90 degree support brackets, M5 bolts are used for this union.
Waiting for ideal weather conditions for the antenna exchange.
Current view of homemade PHQFH.
Results of external mast mounted testing:
Southbound (Night passes) 10th March 2012.
The DCA appears to be more sensitive to locally generated noise and as such, images are poor by comparison to the QFH.
The lower elevation passes were not processed at all due to noise throughout the recording.
However, there does seem to be an improvement over the detail in the images when the interference is absent.
The DCA is more subject to wind, even the slightest breeze starts the assembly swinging. Guys could be used to overcome this but adds to the structure.
Physical post test inspection of the DCA:
The three inch drain pipe does not remain straight at two meters when mounted to the aluminium mast.
The drain pipe was passed over the aluminium mast by 1.5 meters, and then supported with brackets.
There is evidence of cracking on the support plate at the union of the ninety degree brackets. Overtightened bolts are the probable cause.
I think this antenna would be fine for field trip use, away from possible causes of interference.
The length of each dipole has since been reduced to 90% of half a wavelength. The antenna is now in the loft space with a diversity enabled R2ZX receiver.
A study of the interference has highlighted the DCA's susceptibility to a very strong pager frequency at 153.325 Mhz.
Even with the addition of 25dB attenuation the signal RSSI is maximum and can be heard on the 137 - 138 MHz band.
To combat this problem, a notch filter (VHFTN152-156 U.K. Pager Filter) from PAR Electronics was acquired and adjusted to hit the three strongest signals.
The green trace is as received to cover the pager band and the yellow after in-house targeted adjustment.
Update 26th July 2012: A second custom tuned PAR notch filter is now used for the external PHQFH antenna.
The tuning plot for the second filter can be seen below. Pass and reject specs have far exceeded my expectations - very impressive work.
