Having relative poor signals on the local 70cm / 2m channels using this X50 Diamond collinear 70/2 1m antenna at a height of 25m. I needed to investigate what could be wrong.
Measuring the cable only 3W is left for the antenna of the 10W 435MHz output from a IC451E
The cable is 30m type H100 Z50 10mm outside diameter it is semi air-spaced but has an impossible 4mm² solid CU inner conductor not much air left. I preferred the H43 Z75 cable having lower loss and mechanical better ratio in conductor size. My antenna uses SO239 connector UHF type semi air spaced isolator.
3W should be enough for a comfortable noise free FM signal. But only 27km away at PA0A the signal is weak too weak something must be wrong? I made a simple reference dipole of 2x17cm CU wire.
With this dipole the signals where totally worse. So the Diamond does work?
I got curious to look inside. Maybe moisture penetration degraded the antenna.
This antenna is rainwater close in its normal vertical mounting position. But take care if your mast can be tilted downwards then rainwater will flow in immediate the connector base is not sealed.
Inside the fiber tube there is a long copper wire with two coils.
One at the base (picture) with 2 C's and another 40cm off the top with no extra C's
The joint of the 2 C's is the inner pin of the SO239 connector.
The Air-coil underside is soldered to the connector house.
The antenna is connected to the Tap on this coil through a C ( 8.5K) The base coil and its 2 capacitors function as matching network.
Seems a rather small value to me this 8.5pF for 144Mhz. i measured 15pF possible the capacity ages upward.
Measuring the cable only 3W is left for the antenna of the 10W 435MHz output from a IC451E
The cable is 30m type H100 Z50 10mm outside diameter it is semi air-spaced but has an impossible 4mm² solid CU inner conductor not much air left. I preferred the H43 Z75 cable having lower loss and mechanical better ratio in conductor size. My antenna uses SO239 connector UHF type semi air spaced isolator.
3W should be enough for a comfortable noise free FM signal. But only 27km away at PA0A the signal is weak too weak something must be wrong? I made a simple reference dipole of 2x17cm CU wire.
With this dipole the signals where totally worse. So the Diamond does work?
I got curious to look inside. Maybe moisture penetration degraded the antenna.
This antenna is rainwater close in its normal vertical mounting position. But take care if your mast can be tilted downwards then rainwater will flow in immediate the connector base is not sealed.
Inside the fiber tube there is a long copper wire with two coils.
One at the base (picture) with 2 C's and another 40cm off the top with no extra C's
The joint of the 2 C's is the inner pin of the SO239 connector.
The Air-coil underside is soldered to the connector house.
The antenna is connected to the Tap on this coil through a C ( 8.5K) The base coil and its 2 capacitors function as matching network.
Seems a rather small value to me this 8.5pF for 144Mhz. i measured 15pF possible the capacity ages upward.
The other C is parallel over the connector is smaller i could not read the value. i measured 13pF The 2nd coil must be the 2nd Collinear 2m section.
Then in the middle there is an brass tube of about 40cm where the inner conductor goes in but less then 1 inch no galvanic connection. I thought maybe that's the problem... Had to force it open to see. This is just a passive vertical section no galvanic connection to the upper part.
Halve way the wire lead through a 40cm x 10mm diam. brass tube. Both ends are fixed in an 25mm long isolator this section must be a 70cm 1/4 wave i guess although it should be 35cm then not 40.
The mysterious tube in the middle
The antenna seems to work at least better then a simple vertical dipole that i tested at that point. The cable is not super the insertion loss should be about 2.5dB now it is over 3dB
Also i have the Diamond MX7 2 N Duplexer inserted that gives extra but small loss at 70cm But those losses can not explain the weak signal i have at this height and distance?
Then in the middle there is an brass tube of about 40cm where the inner conductor goes in but less then 1 inch no galvanic connection. I thought maybe that's the problem... Had to force it open to see. This is just a passive vertical section no galvanic connection to the upper part.
Halve way the wire lead through a 40cm x 10mm diam. brass tube. Both ends are fixed in an 25mm long isolator this section must be a 70cm 1/4 wave i guess although it should be 35cm then not 40. 
The mysterious tube in the middleThe antenna seems to work at least better then a simple vertical dipole that i tested at that point. The cable is not super the insertion loss should be about 2.5dB now it is over 3dB
Also i have the Diamond MX7 2 N Duplexer inserted that gives extra but small loss at 70cm But those losses can not explain the weak signal i have at this height and distance?
A point of attention are the foam spreaders that have to keep the antenna centred and prevent to touch the fibre dome making annoying sounds. That foam will de-rate and gets brown and crumbly. Also it can absorb water like a sponge.The resonance frequency will change and losses come up. I have replaced it with pieces of white packing foam. Now the the signals of usual 70cm repeaters are strong as before even with the 4dB cable loss.
Comments
a lightning has burned my antenna X30 and I'm looking for the capacity of the two capacitors.
The first one is a 8.5 pF (500 V ?)
Do you know the capacity of the second one?
Thanks a lot
73
Antonio IW0GPW
Sorry for late responce
I could not read the value of the parallel C
my guess it has the same 8.5pF value
If I bring the antenna down again I can unsolder that C and measure it's value
I have had the mast down
measuring only 2.5W left of 10W at 430MHz
H100 33M it is rated 8.8dB 435MHz 100m it should be -3DB # 5W left
the cable may be damaged
refered to site of PA0FRI
they can be replaced by short pieces of coax to make it more weather/ power proof
interesting to know that the gain can be de-rated by aging or lightning ESD damage of the small tuning capacitors
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