Category: Antennas

Galvanised steel wire OCF dipole

I was chatting on the weekend with a new ham who was inspired by an article in Amateur Radio to build an OCF dipole using galvanised fence wire.

I have not seen the article, but he recalled that it recommended single core 1.25mm galvanised wire as quite suited to the task of an 80m OCF dipole. The wire consists of a mild steel core with a thin layer of zinc. Continue reading Galvanised steel wire OCF dipole

Last update: 24th October, 2014, 10:13 AM

Trying to make sense of the VE7BQH Yagi performance tables

VE7BQH publishes a table of Yagi performance data derived from a number of modelling programmes. The table is often used as a definitive reference of the overall merit of an antenna by buyers, makers and students for their own situation.

This article is mainly motivated at better understanding the G/T column of the tables. Continue reading Trying to make sense of the VE7BQH Yagi performance tables

Last update: 26th November, 2014, 6:24 AM

Feed point voltage – W5WSS 7′ dipole

Background

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

Last update: 30th January, 2016, 8:48 AM

Feed point voltage – full wave dipole

Balun designs has a warning to users of baluns on a full wave dipole.

***ONE EXCEPTION!***

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!

References

  • Duffy, O. 2001. RF Transmission Line Loss Calculator (TLLC). VK1OD.net (offline).
  • ———. Jul 2011. Avoiding flashover in baluns and ATUs. VK1OD.net (offline).
Last update: 16th March, 2014, 2:13 PM