Rg 59 coax radiated power ?

drjohsmith

Joined Dec 13, 2021
816
Trying to find a number, and dont know what to look for

IM trying to find out how "good" a coax is at keeping the signal inside !

If I put say 100w "RF" into a length of RG59 coax , into a 50 ohm load,
and transmiter and load are "shielded", all properly conected etc etc,

how much "signal" would get "radiated" out of the coax.

So I know cable loss v frequency , db ,

but hat number in the data sheet would tell us how much power gets leaked out the cable sides?

sagor

Joined Mar 10, 2019
866
Look up the coax loss figures. It will be rated in dB/100 feet or dB /100M. Loss is dependent on frequency as well, low frequencies have less loss. Finally, RG59 is 75 ohm coax, not 50 ohm, and losses will be higher

Example:
https://timesmicrowave.com/calculator/?productId=121&frequency=30&runLength=1&mode=calculate#form

Cable run efficiency is 78% in that example of 50 feet at 30Mhz (assuming 75 ohm source and load). That means 100W will give 78W out in that specific case.
That loss is what "heats" the cable and leaks out via the shield (combined)

nsaspook

Joined Aug 27, 2009
12,297
Trying to find a number, and dont know what to look for

IM trying to find out how "good" a coax is at keeping the signal inside !

If I put say 100w "RF" into a length of RG59 coax , into a 50 ohm load,
and transmiter and load are "shielded", all properly conected etc etc,

how much "signal" would get "radiated" out of the coax.

So I know cable loss v frequency , db ,

but hat number in the data sheet would tell us how much power gets leaked out the cable sides?
What you are looking for is Transfer impedance. Zt
http://www.tscm.com/MIL-HDBK-419A.PDF
8.8.1 Cable Shields.
The effectiveness of a cable shield is a function of two basic interference mechanisms: (1) EM wave shielding
effectiveness and (2) surface transfer impedance, Zt
. As with other shields, the EM wave shielding
effectiveness results from attenuation and reflections and is dependent upon such factors as the type and
thickness of the material used and the number and size of openings in the shield. In addition, cable shields
frequently are connected in such a manner as to carry relatively large currents themselves. Although the
Interfering currents generally flow on the outer surfaces of the shields (skin depth effects), an electric field and
resulting axial voltage gradient is developed along the inner (shielded) conductor (see Figure 8-33). The ratio of
the induced conductor-to-shield voltage per unit length to the shield current is defined as the surface transfer
impedance, Zt
.
The effectiveness of a shield is a function of the conductivity of the metal, contact resistance between strands
in the braid, angle and type of weave, strand sizes, percentage of coverage, and size of openings. Analytical
expressions which define Zt in terms of these parameters are available (8-14). For uniform current distribution
along a cable shield, the resulting Zt can be used to predict the shield effectiveness of the cable knowing the
terminating impedances of the cable. Typically, the cable is several wavelengths long at the frequency of the
impinging field. Thus, the current distribution on the cable sheath varies with length and is a function of its
orientation to the incident wave and to the surroundings. Since the current distribution will be essentially
unpredictable for other than very specialized conditions, the ability to predict shielding effectiveness of the
cable shield through the use of Zt is severely limited.

drjohsmith

Joined Dec 13, 2021
816
Look up the coax loss figures. It will be rated in dB/100 feet or dB /100M. Loss is dependent on frequency as well, low frequencies have less loss. Finally, RG59 is 75 ohm coax, not 50 ohm, and losses will be higher

Example:
https://timesmicrowave.com/calculator/?productId=121&frequency=30&runLength=1&mode=calculate#form

Cable run efficiency is 78% in that example of 50 feet at 30Mhz (assuming 75 ohm source and load). That means 100W will give 78W out in that specific case.
That loss is what "heats" the cable and leaks out via the shield (combined)
Thanks @sagor
its not the amount lost that Im after, as you say a fair amount is "heat"
its the radiated as RF amount Im trying to ascertain.

nsaspook

Joined Aug 27, 2009
12,297
Thank you
not heard of that number, first look , perfect.
thank you
Typically that sort of information is used in the TSCM realm.

A bug hunt.

drjohsmith

Joined Dec 13, 2021
816
Typically that sort of information is used in the TSCM realm.

A bug hunt.
Thank you , not my application but I see where you make the link

nsaspook

Joined Aug 27, 2009
12,297