## Coax connectors and accurate / repeatable measurements

Much is written about the virtues of some types of coax connectors over others.

## 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.

## 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).

## Bushnell Back Track D-TOUR

I purchased a Bushnell Back Track D-TOUR, partly because it appeared to have a useful digital compass.

The digital compass turned out to be a major disappointment . The documented calibration procedure made little difference, it was still commonly more than 10° in error (not due to nearby magnetic objects, or T/M variation).

## IC-7410 tests – 03 – Noise Figure

An article in a series documenting measurements on an IC-7410.

This focuses on receiver Noise Figure.

## Using an attenuator for NoiseLo/NoiseHi in NFM

A common method of making Noise Figure measurements of a receiver is to use a noise generator of known noise power. The output power of the DUT is measured with the generator off (NoiseLo) and on (NoiseHi), a Y factor calculated, and from that Noise Figure is calculated.

## Slot car controller connectors – AU

I ‘needed’ to explore modern controller technology for slot cars.

It is a very long time since I played with slot cars, more the technology (rewinding motors) than the racing, but as a 13 year old, I did a share of that too.