Ferrite chokes on RG 316 cable for GMRS field antenna

I am seeking expertise for a light-weight cable solution to minimize RF noise and common-mode currents on a 25-foot length of RG 316.

APPLICATION
The cable will be attached to a portable-field General Mobile Radio Service (GMRS) antenna which will be hoisted from a tree. GMRS works between these frequencies: 462 MHz - 467 MHz. The RG 316 cable has a Male BNC at the antenna end and a Male SMA at the transceiver/radio end.

QUESTIONS ABOUT FERRITE CHOKES

TYPE OF FERRITE CHOKE

  1. Are Mix 61 ferrite chokes best suited for this application? If not, what is the best balance of cost to performance for this application?

METHOD TO ATTACH FERRITE CHOKE
2. Since a “slide-on” choke won’t fit over the SMA connector, is the snap-on version the best solution? If not, what is recommended?

SIZE OF FERRITE CHOKES FOR RG 316 CABLE
3. Are 3 mm ferrite chokes the right size for RG 316 cable? If not, what is the right size?

PLACEMENT OF FERRITE CHOKES
4. How many ferrite chokes are needed on the antenna side?
5. How many chokes are needed on the transceiver side?

Note: I envision using marine grade heat-shrink tubing to further affix and ruggedize the ferrite chokes to the RG 316 cable.

Thank you for considering my inquiry. I look forward to learning how best to solve for this application.

Greetings,

In response to the enumerated questions:

  1. “Best” involves questions of care-abouts, and only you are a position to understand what you care about and how much. Nickel-Zinc ferrites of flavor # 61 have a time and place, just like their manganese-zinc kinfolk and all of the other magnetic materials out there. This is one resource that may/may not shed some light on relevant considerations.

  2. Again, “Best” is a subjective question. One would seem rather stuck with the snap-on variety however, absent a willingness to reinstall one’s own coax connectors.

  3. “Right” in such a context is a rather flexible concept, once the obvious criterion of needing to clear the cable jacket diameter is met.

  4. “Enough.” This may constitute a number from zero to yes depending on application specifics.

  5. See response to # 4.

A couple things here:

RG316 is lossy, to the tune of 20-ish dB/100’ @ 400 MHz per the representative datasheet excerpted below. If you’re OK with only a few percent of the power that goes into one end of the cable coming out the other then that might not be an issue. If it is, you might want to look at some different cable.

As for the noise/common-mode signal part, the troublesome noise is the stuff that comes in differentially at/near/related to the frequency of interest, for which a common-mode choke isn’t going to do much. Common-mode signals within reason should get rejected by the receiver as a routine matter.

And if this apparatus is going to be tree-borne for any significant length of time, lightning mitigation might be a thing to consider, as it makes for a rather unreasonable common-mode signal…

If one’s serious about the topic, the ARRL Antenna Handbook would be a well-qualified resource to consult. If somewhat less serious, this link should offer some valuable info at lesser cost.

Finally, be advised that non-compliance with applicable rules and regulations will put you at risk of prosecution.

Hi Rick, Thank you for your response and links.

Please allow me to reframe my question. Since RG 316 is (too) lossy, what 25 foot length of cable do you recommend for attaching a field-portable GMRS antenna?

The “care abouts” include:

  • Weight (1.5 - 2 pounds, less is better)
  • Cost (less is better)
  • Signal/Sound Quality (What an engineer would consider practical (obvious?) methods to mitigate RF noise or anything which may degrade signal/sound quality for GMRS frequencies.)

Thank you for advising that Ferrite chokes won’t do much since common-mode signals routinely get rejected by the receiver. I take this to mean this applies to any 25 feet of cable for this field application and not specific to RG 316 cable. Is that right?

IF I misunderstood AND your recommended cable would benefit from using Ferrite choke beads:

  • What mix type is appropriate for this application? (or does it even matter?)
  • What size bead fits your recommended cable?
  • How many beads would you recommend for a 25 foot length?
  • Where on the 25 foot cable would you place the Ferrite beads?

The link to the article on “Specifying a Ferrite for EMI Suppression” was interesting but didn’t provide practical direction. However, I am grateful to identify another possible source of products and expertise.

RE: Lightning - As a field-portable cable attached to a GMRS antenna, this is not a permanent installation. Lightning mitigation for an installation is not a concern.

Thank you again for your timely response and helping me along this journey.

@michael5430

For the cable.
From the searching I have done I have seen recommendations to use LMR-400 cable.
I found the 4613-LMR-400-UF-BM-SM(QTY:8M)TB-ND for with the connectors you indicate.
Weight = 0.068lb per foot
This is 26.2’
1.7816lbs
Link to that item.

Data-sheet Click here

Hi Nathan,

Thank you for providing a link to the LMR-400 cable.

What other cables do you recommend at (~25 feet) which are less lossy than RG316 and less expensive than LMR-400?

Michael