Refurbishing aluminium antenna parts affected by weather / corrosion

Weather is not kind to aluminium antenna parts, often giving rise to corrosion that may result in high resistance joints that then reduce radiation efficiency.

It is good practice to document antenna behavior at installation, and through life, measurements can be compared to that benchmark to possibly reveal changes resulting from corrosion. See Diagnosing a possible antenna problem by comparison with a baseline for more discussion.

WARNING: wear eye protection when using any of the appliances mentioned in this article.

Corrosion preventative compounds

Above is the kind of structure that often develops corrosion in the overlapping / clamped tube sections. They can be recovered by cleaning the corrosion products out, and reassembling them with purpose specific conductive waterproof grease containing irregularly shaped shavings of zinc. The way in which it works is that the sharp particles of zinc penetrate the insulating oxide layer on clean aluminium providing a low R conducting path, and the grease prevents ingress of water and oxygen, so preventing corrosion.

Some forms use a silicon grease base for applications where mineral grease is incompatible with wire insulation, but they are more expensive. Continue reading Refurbishing aluminium antenna parts affected by weather / corrosion

Balanced ATUs – the Holy Grail?

It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”

The word “true” in there bodes poorly!

It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.

Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.

A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.

Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?

Modelling an antenna as a simple two terminal resistance is often naive

in the article A simple transformer model of the Guanella 1:4 balun – some further observations I stated:

Note that a two terminal impedance is a naive representation of many if not most antennas, popular, but a naive over simplification that does not facilitate evaluation of current balance.

An example was a recent posting above that used the model to make assertions about the behaviour of a Guanella 1:4 balun.

This article reports results of two experiments with NEC to model an ‘imperfect’ half wave dipole. It is not exactly resonant, but the main issue is that it is tilted from one end to the other, it is not parallel to the ground surface. Continue reading Modelling an antenna as a simple two terminal resistance is often naive

A simple transformer model of the Guanella 1:4 balun – some further observations

A simple transformer model of the Guanella 1:4 balun discussed a simple model for the operation of the device, but a model that is too simple for most RF baluns. Notwithstanding that, it does expose some interesting issues that are not only valid at lower frequencies, but will also be manifest in an RF balun.

Isolated load

Consider the effect of breaking the connection at the red X, so that we now have  what is often referred to as an “isolated load”. Continue reading A simple transformer model of the Guanella 1:4 balun – some further observations

A simple transformer model of the Guanella 1:4 balun

(Guanella 1944) described a 1:4 balun, of a type often known as a current balun.

From Definition: Current Balun, Voltage Balun:

An ideal current balun delivers currents that are equal in magnitude and opposite in phase.

A good current balun will approach the ideal condition. It will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance (including symmetry).

Common mode current will be small.

If the load impedance is not symmetric, then the voltages at each output terminal will not be equal in magnitude and opposite in phase. (Note that for a truly ‘isolated’ load, one well represented as a two terminal load, the currents MUST be equal in magnitude and opposite in phase, but the voltages may not be equal in magnitude and opposite in phase.)

A simplified model

 

Above is a schematic of the Guanella 1:4 balun as often presented, this is an edited graphic from the ARRL manual, so may be familiar to readers. Continue reading A simple transformer model of the Guanella 1:4 balun

An experimental propagation beacon on 144.385MHz – part 2

An experimental propagation beacon on 144.385MHz laid out plans and some first test results.

This article explores a possible deployment scenario and likely paths over which it may be ‘heard’ using Spectrum Lab or the like to dig the signal out of the noise.

Path

Transmit end

Tx power is 10W with on-off keying (OOK or A1 CW).

Tx antenna system gain ~17dBi.

Receive end

Rx antenna gain is 9dBi.

Rx NF specification is 12dB (4236K).

Above are the results of an ambient noise test. In this case, the noise floor at the antenna is some 7000K, it is probably ~7dB higher than a modest weak signal station. Continue reading An experimental propagation beacon on 144.385MHz – part 2

An experimental propagation beacon on 144.385MHz

An experimental beacon on 144MHz has been deployed for evaluation. The beacon is designed to permit observation of aircraft enhancement propagation by way of a 200+s unmodulated carrier in each 300s cycle. Ident is by very slow Morse code. Necessary bandwidth (ITU-R SM.1138-3) is just under 5Hz, requiring 5Hz receiver bandwidth for ‘crisp’ decoding under weak signal conditions, but 1Hz receiver bandwidth is better for observing aircraft reflections.

There has been long running argument about whether such propagation paths are reflection from hot gasses behind the aircraft, or reflection from conducting skin on the aircraft. With increasing use of reinforced plastic skins, we may observe different response from similarly sized aircraft, depending on the skin, and these differences may be frequency dependent.

An interesting topic for study.

Details:

  • frequency: 144.385MHz, 144.384Hz USB dial freq, 144.385MHZ dial frequency in CW mode on modern transceivers (accuracy should be within 200Hz);
  • power: 20W EIRP (current details: https://vkspotter.com/?action=beacon-item&bid=355), ACT, horizontally polarised, antenna is 20m AGL;
  • modulation: ~5 minute cycle uses A1 Morse modulation (OOK) QRSS1 (1s dits) callsign (VK1OD) followed by key down for the rest of the cycle;
  • location is QF44op.

Continue reading An experimental propagation beacon on 144.385MHz

KB0YH’s STLcalc v2.05

In the light of Small transmitting loop calculators – a comparison a reader asked my thoughts on yet another small transmitting loop calculator, KB0YH’s STLcalc v2.05.

There are lots of small loop calculators published, and yes, I have added to the number. Most are some form of elaboration of formulas published by (Hart 1986), and given ‘imprimatur’ by ARRL (Straw 2007). These formulas are deeply flawed, see Reconciling W5QJR’s loop formulas.

For that reason, my first step in reviewing any small loop calculator is to look for hints of Hart. Continue reading KB0YH’s STLcalc v2.05

Some wooly thinking on Antenna Factor online

Antenna Factor is often given / used as a parameter for an antenna (system).

An antenna with (nearly) constant AF can be quite convenient to simple field strength measurement where the AF value establishes a simple relationship between antenna terminal voltage and the external electric field strength.

Antenna Factor (AF) is the ratio of field strength to antenna terminal voltage for an antenna, dimensionally \({AF}=\frac{E}{V}=\frac{V/m}{V}=1/m\), AF units are 1/m or can be expressed in dB as \(AF_{dB}=20 \log_{10} AF \text{ dB/m}\).

It is lazy practice (though not uncommon) to simply express AF in dB, but wrong.  Continue reading Some wooly thinking on Antenna Factor online

Diagnosis of a 9:1 transformer from NanoVNA plot – part 2

Diagnosis of a 9:1 transformer from NanoVNA plot discussed an example measurement of a 9:1 transformer on a binocular ferrite core. These are often recommended for use with Beverage antennas on 160 and 80m bands, and this was the maker’s application. In that article, I hinted that the core might not be #73 as the maker thought, or wished.

This article reports measurements of a 9:1 transformer wound on a Fair-rite 2873000202 (#73) binocular core. The pic above shows the test fixture. Continue reading Diagnosis of a 9:1 transformer from NanoVNA plot – part 2