08-01-2015, 10:35 AM
(This post was last modified: 08-01-2015, 10:48 AM by Radio Fixer.)
Well Al I struggled a little with the article and may have to read it again. I have never used a transmitter but as you say its also applicable to a receiver (certainly the theory).
What did I get from it: well I now know where the magic 50 Ohms comes from. And once upon a time I worked in military electronics and I can remember the vehicle whips having ATUs. I never got involved with them as I wasn't a transmitter guy but it gives me some understanding of what they were there for.
Recently, I restored an AK 308 and that has an aerial input that can be used called "Doublet" that I hadn't heard of before. I'm sure you know all about it but for those that don't please excuse me quoting a bit from the article written for the BVWS:
[i][i]Doublet (un-tuned dipole) is an American term, not heard by me before and AK sold a “D” Doublet Antenna Kit. The kit comprised coils of wire for the antenna leads and then spacers, to use on the wire, to create a parallel transmission line input to the house and radio set. Visualising this is easy if you think of the FM window aerials that most of us will have tried. Here, a piece of 300 Ohm balanced feeder is opened out at the ends to the desired lengths to make, in this case, a tuned aerial. To make the AK doublet antenna the dipole sections would have been mounted remote from the house, away from mains borne electrical interference, and then the feed in would have been balanced. To do this the kit supplied spacers, standoffs and importantly a dual lightening arrestor. In information given to me the makers say “… noise picked up by one of the two lead ins is balanced out by bucking it against identical noise in the other lead in.”
[/i][/i]
I also have a picture of the kit:
I could write more as I am using a Wellbrook mag aerial and very good it is even just stood on the window sill of the Den. The coax feed (50 Ohms of course) is just wired to the long wire input with no matching. I did have it suggested that I should include a resistor to pad the 50 Ohms out to a few hundred but that to me just seemed like loosing the power in the resistor rather than in the mismatch. Can standing waves be a problem on a broadcast set up? I think not but that's only a gut feeling.
The nice man at Wellbrook did say I could fit a Balun (back to the article again) but said it was probably worth only a couple of dBs.
May experiment when I get it rigged properly away from the house.
Phew! Long one for me
Gary
What did I get from it: well I now know where the magic 50 Ohms comes from. And once upon a time I worked in military electronics and I can remember the vehicle whips having ATUs. I never got involved with them as I wasn't a transmitter guy but it gives me some understanding of what they were there for.
Recently, I restored an AK 308 and that has an aerial input that can be used called "Doublet" that I hadn't heard of before. I'm sure you know all about it but for those that don't please excuse me quoting a bit from the article written for the BVWS:
[i][i]Doublet (un-tuned dipole) is an American term, not heard by me before and AK sold a “D” Doublet Antenna Kit. The kit comprised coils of wire for the antenna leads and then spacers, to use on the wire, to create a parallel transmission line input to the house and radio set. Visualising this is easy if you think of the FM window aerials that most of us will have tried. Here, a piece of 300 Ohm balanced feeder is opened out at the ends to the desired lengths to make, in this case, a tuned aerial. To make the AK doublet antenna the dipole sections would have been mounted remote from the house, away from mains borne electrical interference, and then the feed in would have been balanced. To do this the kit supplied spacers, standoffs and importantly a dual lightening arrestor. In information given to me the makers say “… noise picked up by one of the two lead ins is balanced out by bucking it against identical noise in the other lead in.”
[/i][/i]
I also have a picture of the kit:
I could write more as I am using a Wellbrook mag aerial and very good it is even just stood on the window sill of the Den. The coax feed (50 Ohms of course) is just wired to the long wire input with no matching. I did have it suggested that I should include a resistor to pad the 50 Ohms out to a few hundred but that to me just seemed like loosing the power in the resistor rather than in the mismatch. Can standing waves be a problem on a broadcast set up? I think not but that's only a gut feeling.
The nice man at Wellbrook did say I could fit a Balun (back to the article again) but said it was probably worth only a couple of dBs.
May experiment when I get it rigged properly away from the house.
Phew! Long one for me
Gary
