Frequently Asked Questions

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The ground lug is connected directly to the coax shield.  It is normally located on the coax side of the unit, rather than the side that goes to the antenna. 

It is not mandatory that the ground lug be grounded.  It is just there to make it easier for you if you want to ground the system at that point.  There is really no right or wrong about it because antennas vary so widely.  If you are fighting RFI, we recommend that you try without grounding the lug and, if you still have issues, then try grounding the system at the lug.  Results vary depending upon the specific situation. 

It is a straightforward tradeoff between power handline capability and mounting flexibility.  The vented units need to be mounted with the vents generally pointing down.  The sealed units are weatherproof and can be mounted in any orientation, or "hung" in the air or laid upon the ground.

The vented units handle about 80% more power than the sealed units.  Good ventilation makes a big difference in the temperature of the ferrite core.

All coax fed antennas generate some level of common mode current on the coax shield.  This current reduces the current that actually gets to the antenna and thereby reduces signal strength on both transmit and receive.  Common mode current flowing on the shield can also cause RFI and increase your noise floor on receive.  

But if you are NOT experiencing RFI, then you may be able to operate satisfactorily without any Common Mode Chokes in your system.  The gain and pattern of your antenna will still be changed by whatever common mode current there is (because the shield is radiating), even if it is not large enough to cause RFI.  But if you are happy with the antenna's performance and you have have no RFI problems, then you could be fine without any Common Mode Chokes.

If you have no RFI problems, then the issue of whether or not Common Mode Chokes are needed comes down to "what type of operator or person you are".  If you are a minimalist, then you can get by without a choke.  However, if you are the sort of person who wants the best possible performance of your station, then the use of Common Mode Chokes is generally considered essential.  

Absolutely, assuming that the chokes are good enough on the frequencies of interest.  

 Common Mode Choke presents a relatively high impedance to common mode current, which "encourages" the current to flow into the antenna rather than down the outside surface of the coax shield.  

There are two "figures of merit" that one can look at to determine whether or not a particular Common Mode Choke will effectively eliminate RFI:

    1) Common Mode Rejection Ratio (CMRR), and

    2) Choking Impedance.

A choke with higher CMRR (on the frequencies of interest) will always outperform one with a lower CMRR.  

A choke with higher choking impedance (on the frequencies of interest) will almost always outperform one with lower impedance.  

CMRR and choking impedance are not independent.  If one is higher, then so will be the other one.  They are just two ways of measuring the effectiveness of the inductor that is the Common Mode Choke.

Probably, but It depends on how the noise is getting into your system.

Noise that is picked up directly by your antenna will not be lowered by a Common Mode Choke.  But noise picked up by the coax shield WILL be reduced by Common Mode Chokes.

If you are experiencing RF Interference (RFI) then it is likely that your coax is also picking up local noise from household devices.  Unfortunately, many modern electronic products use switching power supplies, some of which generate electrical noise. Even USB phone chargers can cause a lot of noise.

Local noise gets into your receiver via the same path that causes RFI, namely common mode current flowing on the coax shield.  

However, it is possible that noise can be picked up on the coax shield, and raise your noise floor, even if you are not experiencing RFI.  This happens when your household contains no devices that are sensitive to RFI.  The RFI is there, it's just not causing any problems.  But local noise can still induce common mode current on the coax shield of your feedline and get into your receiver, raising the noise floor.