View previous topic :: View next topic |
Author |
Message |
andypandysdirtylandy Gate Opener
Joined: 23 Feb 2007 Odometer: 44
|
Posted: Sat May 02, 2009 2:55 pm Post subject: Coax ?
|
|
|
Can anyone tell me if RG 59 B/U is ok to use on a cb set up ?
|
|
Back to top |
|
|
|
Kitesurf Difflock Royalty
Joined: 28 Feb 2005 Odometer: 14197 Location: Luton, Beds
1994 Toyota Surf
|
Posted: Sat May 02, 2009 3:58 pm Post subject:
|
|
|
It is no good for CB. It is 75 Ohm. You need RG58 which is 50 Ohm.
There is a cheat that can be done to transform from 75 to 50 Ohm but you would need to be able to calculate an electrical 1/4 wavelength of cable, which may end up too short on a vehicle installation. Multiples of a 1/4 wavelength would work but would leave you with extra cable coiled up in the vehicle. It is far easier to use the right stuff.
__________________________________ Motorist who drove his 4x4 up Snowdon sentenced to appear on Top Gear. |
|
Back to top |
|
|
northumberlad Guest
|
Posted: Sat May 02, 2009 4:04 pm Post subject:
|
|
|
RG-58/U is what you want.
|
|
Back to top |
|
|
andypandysdirtylandy Gate Opener
Joined: 23 Feb 2007 Odometer: 44
|
Posted: Sat May 02, 2009 4:54 pm Post subject:
|
|
|
I'm co phasing two whips and I've been told I need 75 ohms coax and I had this stuff knocking about but I think its for video rather than audio thus the question
|
|
Back to top |
|
|
northumberlad Guest
|
Posted: Sat May 02, 2009 6:10 pm Post subject:
|
|
|
have heard of people using 50ohm & 75ohm for phased arraysif you use 75ohm, only use it for the co-phase harmes (the theory for using 75 ohm cable seema to be that the two antemas in paralel present a 70ohmish load) the feeder back to the radio should still be 50ohm
|
|
Back to top |
|
|
mrcheese Mud Obsessed
Joined: 14 Oct 2003 Odometer: 6339
|
Posted: Wed May 06, 2009 8:55 am Post subject:
|
|
|
This link may be of interest.
http://www.bellscb.com/cb_radio_hobby/antennas/antarray.html
Not sure if you will be able the two antennas far enough apart for it to work on vehicle. I am sure that I have read about spacing closer than 1/4 wavelength, but that will change the feed impeadance to an even lower value
|
|
Back to top |
|
|
northumberlad Guest
|
Posted: Wed May 06, 2009 9:41 am Post subject:
|
|
|
the 'use 18" (1 wavelength-ish) of cable' always makes me a bit wary of advice because it't a cheat to get the VSWR down by making relections coincident with the outgoing signal, you still have reflections from impeadance mis-matches they are just hiding. the rest of it seems to make sense from what i can remember fro collage It's just the trying to cheat the VSWR that annoys me
|
|
Back to top |
|
|
mrcheese Mud Obsessed
Joined: 14 Oct 2003 Odometer: 6339
|
Posted: Thu May 07, 2009 1:05 pm Post subject:
|
|
|
It is not a cheat if done properly. The first assumption is that the antenna is of the correct lenght otherwise the the feed point impedances are not what you are expecting. Think of it as impeadance transformation. The remaining 50ohm cable back to the radio is then correctly matched.
What is more of a cheat is an antennna matiching unit at the radio end. This allows the radio to see the correct 50 ohms. That means the coax could actually be have a very high VSWR and therefore be lossy.
|
|
Back to top |
|
|
. Difflock Royalty
Joined: 19 Jun 2002 Odometer: 40007 Location: Northern Ireland's Gold Coast
2009 Land Rover 110 CSW
|
Posted: Thu May 07, 2009 1:44 pm Post subject:
|
|
|
mrcheese wrote: | This link may be of interest.
http://www.bellscb.com/cb_radio_hobby/antennas/antarray.html
Not sure if you will be able the two antennas far enough apart for it to work on vehicle. I am sure that I have read about spacing closer than 1/4 wavelength, but that will change the feed impeadance to an even lower value |
Quote: | So the final question is : "why do I want to do this?" |
Personally I don't see the point!
__________________________________ Bert the Defender 110 XS - because it's Cool (work it out yourself!)
Lolita the Lightweight
???? the V8 90 CSW
Suzuki DL1000 V-Strom - yes that's right, I have a Zook! |
|
Back to top |
|
|
nivapilot Guest
|
Posted: Thu May 07, 2009 6:07 pm Post subject:
|
|
|
. wrote: | mrcheese wrote: | This link may be of interest.
http://www.bellscb.com/cb_radio_hobby/antennas/antarray.html
Not sure if you will be able the two antennas far enough apart for it to work on vehicle. I am sure that I have read about spacing closer than 1/4 wavelength, but that will change the feed impeadance to an even lower value |
Quote: | So the final question is : "why do I want to do this?" |
Personally I don't see the point! |
Isn't this what the american truckers do?
Fit two antenna on the outer mirror stays, to get the signal to go front and rear further, so that they don't get too much from either side.
|
|
Back to top |
|
|
|
Kitesurf Difflock Royalty
Joined: 28 Feb 2005 Odometer: 14197 Location: Luton, Beds
1994 Toyota Surf
|
Posted: Thu May 07, 2009 11:34 pm Post subject:
|
|
|
I understand what you want to do now, which is to make a co-phasing harness. I too don't really see the point, unless it is for a vehicle where you are going to be doing a lot of motorway driving.
Another problem is that you have to take the velocity factor of the coax into account, so an electrical 1/4 Wave will be, 75/fMHz x VF = Electrical Length, which is 75/27.405 x 0.66 = 1.8M. The lead out from your CB can be a random length of 50 Ohm coax going to a T-Piece with a length, as above, going to each antenna. The best way to set it up would be to set the SWR for each antenna, while it is being fed with a single 50 Ohm coax, then add your T-Piece and connect your 75 Ohm phasing lines to the antennas. For a maximum of 3dB gain, which represents just one S Point, it is a lot of work. As has been pointed out above, antenna spacing is also going to have an effect on the impedence presented to the transmitter.
I have done it for the lower frequency ham bands (80 and 40M) where it is more of an asset for RX rather than TX, although one S Point can make all the difference, when trying to work New Zealand on those bands!
__________________________________ Motorist who drove his 4x4 up Snowdon sentenced to appear on Top Gear. |
|
Back to top |
|
|
mrcheese Mud Obsessed
Joined: 14 Oct 2003 Odometer: 6339
|
Posted: Fri May 08, 2009 10:02 am Post subject:
|
|
|
Phased array verticals are great fun on the lower HF bands. But a four element system uses an awful lot of coax.
As Phil says, probably more effective on receive as they can be used to null out noise.
|
|
Back to top |
|
|
|