The Carolina Windom is very popular with modern hams, and at the same time is commonly the discussion of problems in online fora.
The question is whether it is its popularity that is the reason for cries for help, or whether there is something inherently high risk in the ‘design’.
The original Windom
The first type is the classic ‘original’ Windom with single wire feed which folk lore explains as a horizontal wire being tapped at a point where Z matches the vertical ‘single wire’ feeder, that there is not a standing wave on the feed line, and that it does not radiate. Traditional characterisation as a single-feeder Hertz
denies the existence of the vertical radiating element.
It is a folly to designate the vertical wire as a non-radiating feeder, it carries an RF current that contributes to radiation just like current on the horizontal wire does.
(Straw 2007) observes of the original Windom:
Because the single-wire feed line is not inherently well balanced and because it is brought to the operating position, “RF in the shack” and a potential radiation hazard may be experienced with this antenna.
This common mode current issue motivated variations on the original design, and the Carolina Windom is one in the series searching for a solution.
Carolina Windom
The Carolina Windom is described in (Straw 2007) who attributes it to Edgar Lambert (WA4LVB), Joe Wright (W4UEB), and Jim Wilkie(WY4R) who published a variation on the ‘original’ Windom.
Above, Straw’s diagram of the Carolina Windom. The diagram gives no guidance on key implementation details such as height, and the nature of the common mode current path to ground via the RG-8X to Antenna Tuner
path. There is no ‘standard’ installation, but these elements can have significant impact on behaviour of the antenna system.
NEC model
An NEC-4 model was developed making some assumptions to extend the detail above to a more complete implementation that could be modelled and provide some insight into issues that arise.
Two options are explored:
- low Q current balun; and
- high Q current balun.
Low Q current balun
Assumptions:
- frequency=7.1MHz;
- height=10m;
- good 4:1 voltage balun at dipole feed point;
- current balun |Zcm|=3000Ω, Q=1 (in line with the fashion for ‘resistive’ baluns);
- feed line falls vertically to 3m height, then horizontally for 4m under the long side of the dipole, then falls vertically to ground.;
- equivalent ground resistance 10Ω.
Above shows the current distribution on the structure.
At 100W continuous input, common mode current Icm into the ground (ie at the shack) is 0.39A. This is very high (20dB higher than good) and likely to cause EMC issues in a lot of situations.
Also concerning is that the dissipation in the current balun is 25.5W, 25% of the average input power. This is not only poor efficiency, it is unsustainable with practical baluns at average input power much over about 50W.
The scenario turns out to expose quite poor behaviour with the low Q balun, and changes may make it better or worse. The point is that the model does expose effects that would lead to the symptoms commonly reported of Carolina Windom implementations.
High Q balun
Assumptions:
- frequency=7.1MHz;
- height=10m;
- good 4:1 voltage balun at dipole feed point;
- current balun |Zcm|=2000Ω, Q=100 (in the style of Radioworks
The Line Isolator provides a large inductive reactance at the insertion point (the action is similar to an RF choke)
); - feed line falls vertically to 3m height, then horizontally for 4m under the long side of the dipole, then falls vertically to ground.;
- equivalent ground resistance 10Ω.
Above shows the current distribution on the structure.
(Radioworks nd), a seller of ready made Carolina Windoms makes claims of the current balun (line isolator in their terminology):
The LINE ISOLATOR™ effectively separates the transmission line portion of the cable from the radiating portion. Vertical radiator length is predictable and repeatable and it performs double duty as part of the matching system.
Their claims depend on undisclosed properties of their current balun, but are not supported by the model used here which shows that whilst the current balun might influence the common mode distribution, the current at the balun is a minimum (0.16A), the balun does not prevent the development of a standing wave on the transmitter side which can result in higher current conducted into the shack.
At 100W continuous input, common mode current Icm into the ground (ie at the shack) is 0.43A. This is very high (20dB higher than good) and likely to cause EMC issues in a lot of situations.
Dissipation in the high Q current balun is quite low at 0.5W, 0.5% of the average input power.
The scenario turns out to expose quite poor behaviour with the high Q balun, and changes may make it better or worse. The point is that the model does expose effects that would lead to the symptoms commonly reported of Carolina Windom implementations.
Conclusions
The Carolina Windom:
- may have significant common mode current on any part of the feed line, current that:
- contributes to radiation on transmit;
- on receive contributes to signal and more so, local noise pickup; and
- gives rise to EMC problems;
- dissipation in the common mode balun may be relatively high and unsustainable with practical components at moderate average power;
- the low Q balun leads to very slightly reduced Icm in the shack (9%) compared to the high Q balun, but has a serious dissipation problem;
- the general fashion for ‘resistive baluns’ may not be sound;
- measures that are assumed to eliminate common mode current might not be efficient or effective;
- actual behaviour is very dependent on implementation topology and components.
References
- Radioworks. nd. What make the CAROLINA WINDOM so good? http://www.radioworks.com/ccwsogood.html (accessed 02/10/2015).
- Straw, Dean ed. 2007. The ARRL Antenna Book. 21st ed. Newington: ARRL.