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
It is more than 10 years since I designed the PIK (PIC Iambic Keyer), a semi automatic Morse keyer (Duffy 2000).
This article looks at the operation of the auto-space feature of PIK. Continue reading PIK and auto-space
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
A correspondent is designing a high performance transverter from 144MHz to 28MHz to be used with a Elecraft K3 transceiver.
A question that arises is just how good should the performance of the transverter be, what Noise Figure and Gain should it have without having performance that cannot be fully realised due to external noise. Continue reading High performance 144MHz transverter design workup – noise and gain analysis
NFM has been updated to v1.18.0.
It includes for user convenience, a noise measurement uncertainty calculator based on the discussion of uncertainty of the noise sampling process at (Duffy 2007b) and the calculator at (Duffy 2007c)
Continue reading Update for NFM software (v1.18.0)
Folk often ask for instructions for assembly of so-called clamp type coax connectors.
This article shows such an N type connector, but the techniques are applicable to different connectors with the same type of cable / braid attachment. Continue reading Assembling clamp type coax connectors
An article in a series documenting measurements on an IC-7410.
This focuses on receiver Tx power and SWR meter.
Continue reading IC-7410 tests – 04 – Tx power & SWR meter
Much is written about the virtues of some types of coax connectors over others.
Continue reading Coax connectors and accurate / repeatable measurements
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).