Off centre fed (OCF) dipoles are very popular with Hams on HF.
Many claims are made for variations of the theme, but the essential common feature is that the offset feed point provides a feed point impedance that is not too unsuited to 50Ω coaxial feed line at the fundamental resonance of the dipole and half wave harmonics. Continue reading OCF dipoles
There is a common Rule of Thumb that to be effective, the magnitude of common mode impedance of a Guanella current balun in an antenna system should be at least 500Ω (Reisert 1978).
This article proposes a possible origin for this thinking, and discusses the validity of the underlying model. Continue reading Baluns – Rule 500
One sees simplistic application of ohms law to antenna balun problems frequently in online forums, but is the technique valid? Continue reading Using Ohms law on antenna baluns
ON9CD (Vandonselaar 2002) gives an expose on baluns for Hams.
Above is the first diagram he gives in support of his explanation. Continue reading ON9CVD on baluns
W5WSS describes his antenna at (eHam 2014). It is essentially a shortened dipole with capacity hats for 20m.
The configuration appears from several postings to be this shortened dipole with a Balun Designs 1115du balun at the center and an adjacent LDG Pro 200 automatic ATU.
Continue reading Feed point voltage – W5WSS 7′ dipole
Balun designs has a warning to users of baluns on a full wave dipole.
It must be pointed out that a 1:1 balun should never be used on the second harmonic of a half-wave center-fed dipole fed with coax (like an 80 meter dipole being used on 40 meters). The impedance can be as great as 10,000 ohms creating very high voltages which can bring about voltage breakdown and/or excessive heating. This exception ONLY applies to Coax Fed HALF WAVE CENTER FED DIPOLES WHEN USING A 1:1 BALUN AT THE FEEDPOINT.
Whilst differential voltage can be an issue in antenna systems (Duffy 2011), the warning above is a bit dramatic for this case.
Firstly, it is very difficult to measure the impedance of a full wave centre fed dipole in the worst case, but modelling suggests it is unlikely to have an impedance at resonance greater than about 4200+j0Ω.
Lets suppose there is a balun located at the feed point of an 80m half wave dipole, and the antenna is fed with 25m (~80′) of RG58C/U feed line. Using TLLC, the transmission efficiency of that section of line at 7MHz with load of 4200+j0Ω is just 9.4%.
If we have a 100W transmitter, we might get 90W out of the ATU in this scenario, and 9.4% or 8.5W of that reaches the feed point.
It is a simple matter to calculate the RMS voltage as V=(P*R)^0.5=190V, or 270Vpk. This is not going to strain any balun!
They outcome here is due to the extreme loss on the coax under very high standing waves results in very little power reaching the balun anyway.
This is one of those cases that if the antenna was half as long, the system would be ten times as good!
- Duffy, O. 2001. RF Transmission Line Loss Calculator (TLLC). VK1OD.net (offline).
- ———. Jul 2011. Avoiding flashover in baluns and ATUs. VK1OD.net (offline).
Multi band antennas are compromises more so than most mono-band antennas, and part of that compromise is lower efficiency. Often the lower efficiency aspect is accepted without understanding.
Continue reading Unun on 13m (43′) vertical
The G5RV Inverted V antenna system at VK2OMD is fed with 9m of home made open wire transmission line using 2mm diameter copper wires spaced 50mm giving a line with characteristic impedance of 450Ω. (Varney 1958) described the tuned feeder configuration of his popular G5RV antenna system.
Continue reading VK2OMD G5RV with tuned feeder – line loss
I described a method for designing antenna systems to avoid excessive voltages in baluns and ATUs at (Duffy 2011) .
This article reports post implementation measurements of an antenna system designed using that method and using a G5RV Inverted V with tuned feeder and ATR-30 ATU with integral 1:1 current balun. The tuned feeder is a home-made line section of 2mm diameter copper conductors spaced 50mm, and 9m in length. An additional 0.5m of 135Ω line connects from the antenna entrance panel to the ATU.
Continue reading ATU voltage verification
This article explores the loss that may be encountered in an ATU in a practical setting.
The load is a G5RV with tuned feeders operating at 3.6MHz. The tuned feeder is 9m of open wire line of characteristic impedance 450Ω, and the impedance seen by the ATU is around 40-j150Ω, this is not a particularly onerous load.
Continue reading A look at internal losses in a typical ATU