Exploiting waveguide mode of the loop conductor in a small transmitting loop

Small transmitting loop enthusiasts search for explanations of why their antennas are so fantastic.

One of those fantastic explanation is from KK5JY:

I have spent some time thinking about this discrepancy, and how to account for it  within the typical ham home-made loop. This is not to say that I am asserting this as correct, but I suspect there are straightforward reasons why the efficiency of a small loop of typical construction could be better than the classic formulae predict.

One simple possibility has to do with construction. Many loop designs, mine included, use open-ended copper tubing for the radiating element. Mechanically, this means that the loop itself actually has two conductors, wired in parallel. One is the outside of the loop conductor, and one is the inside of the loop conductor. The reason for this is skin effect. Anybody who has run high power RF into a coaxial cable that is poorly matched to a balanced antenna is familiar with the “feedline radiation” effect, where the shield of the coaxial cable forms two conductors, with current flowing on both. In the loop case, The outer and inner surfaces of the loop conductor are connected together at the ends, so the two conductor shells carry current in parallel. Depending on the difference in diameter of the two surfaces, the effective increase in surface area can be almost 100%, roughly doubling the surface area of the main element. “But the inner conductor is shielded from the environment by the outer conductor,” someone might object. This is true for the electrical field, but not the magnetic field, which just happens to be the largest component of the EM near-field created by this type of antenna. A small loop is driven almost completely by the magnetic field generated by the driven element, and the lines of magnetic flux cut both the inner and outer surfaces of the main (large) loop, inducing current flow into each one, independently, and the two are able to create a combined magnetic field around the antenna.

Continue reading Exploiting waveguide mode of the loop conductor in a small transmitting loop

Last update: 12th June, 2017, 9:01 AM

IC-7300 S-meter calibration accuracy

This article documents measurement of the calibration of an IC-7300 S-meter in SSB mode using a continuous sine wave at 1kHz tone frequency.

Convention

There has been a long standing convention that S-meters are calibrated for 50μV in 50Ω to be S9, and S-points laid out at 6dB per S-point. IARU Region 1 formalised this with Technical Recommendation R.1 which defines S9 for the HF bands to be a receiver input power of -73 dBm (equivalent to 50μV in 50Ω).

IC-7300 measurement

A test was conducted where a Standard Signal Generator was connected to the receiver and slowly increased from -125dBm in steps of 1dB and the point at which the S-meter display segments lit was noted.

Above is a chart of the error between the S meter indication and the value per IARU Region 1 Technical Recommendation R.1.  Continue reading IC-7300 S-meter calibration accuracy

Last update: 10th June, 2017, 8:17 PM

Kenwood R-5000 thermals

I have a Kenwood R-5000 that is now 30+years old and warrants a check of its health.

R-5000s are infamous for VCO problems, the early production used ‘yella glue’ to stabilise the VCO components and that decomposed into corrosive components that damage the electronic parts. Repair is not usually economically rational.

This is one of the later model R-5000s that used the hard white adhesive which has remained stable.

The R-5000 is built on phenolic PCB and operates at relatively high temperature for a simple receiver, reflecting the power consumption of synthesisers of the 1980s.

 

Above, the case temperature is up to 20° above ambient over the power transformer (upper right of pic).  Continue reading Kenwood R-5000 thermals

Last update: 11th April, 2022, 7:22 AM

Estimating the voltage impressed on the tuning capacitor of a small transmitting loop

The ‘net abounds with calculators for design of small transmitting loops (STL), and most estimate the voltage impressed on the tuning capacitor. Most of these calculators give an incorrect estimate.

This article describes a measurement based approach to estimating the capacitor voltage for a STL.
Continue reading Estimating the voltage impressed on the tuning capacitor of a small transmitting loop

Last update: 6th June, 2017, 11:50 AM

A QRP small transmitting loop evaluation

The ‘net abounds with articles describing easy to build low cost small transmitting loops (STL).

This article describes measurement of a STL for 4MHz using RG213 coaxial cable for the main loop and its tuning capacitance, and a smaller plain wire loop for transformation to 50Ω. Continue reading A QRP small transmitting loop evaluation

Last update: 6th June, 2017, 10:27 AM

Precise RF small transmitting loop

Precise RF have announced two small transmitting loops for amateur radio, this article looks at the Precise High Gain Loop.

Description

The antenna is described at (Precise RF 2017).

Above is an extract from a table in the brochure comparing the subject antenna to some others.

On a quick scan, the standout figure is gain of 2.8dBd presumably at a loop height of 4.57m (15′), and without qualification of frequency. Elsewhere in the brochure there is a note that 80m requires an optional ‘resonator’… presumably a larger loop.

Lets review the meaning of dBd

The ITU Radio Regulations (ITU 2012) gives us a definition for antenna gain that captures the meaning of dBd that is accepted by most regulators and industry world wide.  Continue reading Precise RF small transmitting loop

Last update: 5th June, 2017, 8:26 AM

The sign of reactance – SM6WHY’s take

As the popularity of low cost, low end antenna analysers increases, client software appears to enhance the capability of the analyser.

The SARC-100 is one of these low end analysers, it and its many close derivatives are marketed under various model names.

The sign of reactance discusses a major weakness of these and many other low end instruments in that they do not ‘measure’ the sign of reactance, displaying the magnitude of reactance and leaving it to the user to solve the sign problem.

SM6WHY is one of the many who have produced software for the SARC-100 that purports to solve the sign of reactance problem. He gives this graphic on his website to demonstrate the capability of his software used with a SARC-100 (which does not sense the sign of reactance).

Above is part of the graphic he offers. Though the image is poor quality, the VSWR plot appears smooth and quite typical of that which might be obtained by measuring an antenna system near its VSWR minimum.

However the accompanying Smith chart plot which has points plotted with both negative and positive reactance is inconsistent with the VSWR plot and appears flawed.  Continue reading The sign of reactance – SM6WHY’s take

Last update: 28th May, 2017, 9:49 AM

A search for some mid power white wide angle LEDs

I have a project which needs some mid power (~3W) white wide angle (120+°) LEDs.

The obvious source ie eBay which means running the gamut of Chinese sellers, sellers who rarely understand the product they sell and probably expect the same of buyers.

Buying electronic components on eBay

Component sales tend to fall into categories:

  1. those with headline descriptions that have very brief description of characteristics; and
  2. those whose descriptive content claims well known part numbers for which datasheets can separately be found;
  3. those with detailed specifications offered.

In the case of category 1, it is very hard to have confidence that the components will deliver required performance, and headline descriptions on eBay are often used as competitive search keywords and do not apply to the goods on offer. These are probably best skipped unless they are the only option.

Category 2 provides a better option, and the question then on delivery is whether the goods are compliant with the part number offered. There is a considerable risk of counterfeit or fake parts that are not equivalent to the claimed part number, even where brand names are cited.

The third category can provide suitable product, but it takes some leg work, more than ‘due diligence’ to check the description for consistency and form an idea about its reliability, fit to the requirements and then value for money, seller reputation etc. This can be a lot of work for a few dollars worth of parts, but is a better option than category 1.  Continue reading A search for some mid power white wide angle LEDs

Last update: 26th May, 2017, 5:32 PM

Shunt matching a loaded HF whip – discussion

Shunt matching a loaded HF whip with just a VSWR meter gave a direct answer and supporting explanation to an online poster’s question about optimising an 80m loaded mobile vertical with shunt matching, specifically the inductor needed and an adjustment procedure.

The original poster clearly had the impression that this improvement of the original VSWR=1.3 would make a large difference.

The only other option for me is to remove the shunt and set my swr back to 1.3:1 and not be able to communicate.

Continue reading Shunt matching a loaded HF whip – discussion

Last update: 24th May, 2017, 7:57 PM