Jump to content

BNC cable lengths


Recommended Posts

Hi BR faithful,


When using an antenna distro for my radio microphones, should I take into consideration the length of the bnc cable in regards to wavelength.


Is there a rule of thumb on this, or a ratio to bear in mind.


Many thanks




Cable length isn't really wavelength dependent, but you should take into consideration the loss in the cable, which is somewhat frequency (wavelength) dependent if you are thinking about lengths greater than about 25' (8m) . There will be more loss at higher frequencies, and there will be more or less loss depending on cable type and length. In the UHF frequencies that most of our RF mics operate you should be fine below 25' with anything, although RG58 is the weakest of the common cable types. Even 75Ω RG59 is better than RG58. You should try to stick to 50Ω antenna cable, although like speakers, the 50Ω impedance varies some based on frequency since the antennas at least are wide band devices and not tuned for a specific frequency, and AFAIK none of the common receivers have an exact 50Ω input impedance. Matching impedance is good, but generally less critical than cable type choice. Low loss cable like RG213 is pretty good, with a loss of about 6.5dB/100' at 700MHz. If you will be using long cables, like 100' you should get a lower loss cable like the double shielded LMR-400 which has a loss of about 3.5dB at 700MHz. Good antenna choice and placement are also important. You can calculate the RF gain/loss of your antenna/cable system to get a relative idea of how it relates to a receiver with a 1/4 wave whip located where the remote antenna is.



Link to comment
Share on other sites

Only time you really need to be thinking about cable length (apart from in a sense of losses, as has already been mentioned above) is if you have unterminated lengths of cable within your system. Particularly bad news is a T connector with an unterminated length coming off of it - this seemingly harmless length of cable suddenly becomes a reasonably wide notch filter. They're very easy to unwittingly build in to racks as passive splits to feed other racks!
Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.