AE7PD’s transmitting loop measurements

AE7PD documented his measurements of a 3.16m perimeter circular transmitting loop, 1.8m centre height above ground, that he made using 16mm copper tube and a split stator tuning capacitor:

AE7PD gives the radiation efficiency on 20m as 30.5% or -5.2dB.

I present here an alternative analysis of the antenna as measured on 20m.

Assuming the measurements were made with the antenna clear of disturbing conductors etc, and that 5/8″ tube means 16mm OD.

The key measurements were:

  • centre frequency 14.165MHz, VSWRmin=1.0;
  • VSWR=2.62 bandwidth 22kHz.

A NEC-4.2 model of the antenna at 14MHz was built and calibrated to the measured half power bandwidth (22kHz). Model assumptions include:

  • ‘average’ ground (0.005,13);
  • Q of the tuning capacitor = 2000;
  • conductivity of the loop conductor adjusted to calibrate the model half power bandwidth to measurement.

Note that the model may depart from the actual test scenario in other ways.

Above is the VSWR scan of the calibrated model, the load is matched at centre frequency and half power bandwidth is taken as the range between ReturnLoss=6.99dB points. Continue reading AE7PD’s transmitting loop measurements

Findling & Siwiak 2012 measurements of an Alexloop – discussion

I mentioned in Findling & Siwiak 2012 measurements of an Alexloop issues with their efficiency calculation.

Above is an extract from (Findling & Siwiak 2012).

(Siwiak & Quick 2018) give an equivalent circuit of lossless loop structure in free space.

When tuned to resonance, the response is simply that of a series RLC circuit where R=Rr (the radiation resistance) which is dependent on frequency, but varies very slowly with frequency compared to the net reactance X.

Above is a NEC simulation of such a loop. Continue reading Findling & Siwiak 2012 measurements of an Alexloop – discussion

G3CWI 2018 measurements of an Alexloop Walkham

Richard, G3CWI, measured the impedance and bandwidth of a Alexloop Walkham, a popular small transmitting loop (STL). The antenna was situated in the clear at 1.65m centre height above natural ground.

The key measurements were:

  • centre frequency 7.014MHz, |Z|=51Ω, VSWR=1.1;
  • VSWR=3 bandwidth 16.2kHz.

The step size of the analyser prevented measurement exactly at resonance, but R changes very closely with frequency near resonance so we can estimate it quite well. The above figures can be used to find R close to resonance.

Within the limits of measurement error, we can say that R at resonance should be very close to 51Ω, and VSWRmin close to 1.02. Continue reading G3CWI 2018 measurements of an Alexloop Walkham

Findling & Siwiak 2012 measurements of an Alexloop

(Findling, A & Siwiak 2012) documented measurements they made of a popular small transmitting loop (STL), an Alexloop Walkham.

Now Alexloops seem to have undergone some evolution, and there does not seem to be a clear list of model names or numbers with features or specifications, so to some extent the antenna is a little non descript.

The article did not document the environment of the test antenna, but Findling explained in correspondence that it was relatively clear of conducting structures and about 1.2m above natural ground.

A NEC-4.2 model of the antenna at 7MHz was built and calibrated to their measured half power bandwidth (19kHz). Model assumptions include:

  • ‘average’ ground (0.005,13);
  • Q of the tuning capacitor = 1000;
  • conductivity of the loop conductor adjusted to calibrate the model half power bandwidth to measurement.

Note that the model may depart from the actual test scenario in other ways, it is challenging to glean all the data that one would like from the article.

Above is an extract from (Findling, A & Siwiak 2012).

Above is the VSWR scan of the calibrated model, the load is matched at centre frequency and half power bandwidth is taken as the range between ReturnLoss=6.99dB points. Continue reading Findling & Siwiak 2012 measurements of an Alexloop

Single turn coaxial loop resonator analysis

Recent discussion online of a purported commercial HF small transmitting loop (STL) was challenged in analysing the structure, questioning whether such a connection was ‘correct’.

The STL used a main loop resonator and a separate small auxiliary loop for the 50Ω feed, a very common arrangement.

The main loop is a coaxial cable with, in this case, a tuning capacitor inserted between the inner conductors at each end. Above is a diagram of the main loop. Continue reading Single turn coaxial loop resonator analysis

WW1WW’s matching transformer for an EFHW

At PD7MAA’s BN43-202 matching transformer for an EFHW I gave an estimate of the core loss in PD7MAA’s transformer.

An online expert questioned the analysis and later measurements, and proposed his own transformer design as evidence.

Notably, his transformer uses #61 material and a larger binocular core, a Fair-rite 2861006802 with 2t for a nominal 50Ω primary, giving loss measurements at 7MHz of 0.08dB. Note that the confidence limits of that loss measurement because of the way in which it was obtained (eg a 1% error in the 1120Ω load resistor contributes 0.043dB error to the result), but the measurements do suggest that the loss is probably very low.

Though the loss is low and Return Loss is high at 7MHz, the limits for ReturnLoss>14dB (VSWR<1.5) is 5-18MHz. With compensation, that range may be changed.

Prediction

Lets apply the method laid out at PD7MAA’s BN43-202 matching transformer for an EFHW.

The best Fair-rite data I can find quickly is a chart of the impedance of a one turn winding.

Scaling from this graph, Xs is close of 35Ω at 7MHz, so lets used that to derive some basic parameters for the core. Continue reading WW1WW’s matching transformer for an EFHW

AIM 914 produces internally inconsistent results

 

AIMuhf

A new release, AIM914 appeared recently.

In the common theme of one step forward, two steps backwards, this version has defects that were not present in AIM910B.

Let’s review the internal consistency of this part of the display screen.

Most of the values given above are calculated from a single measurement value, and should be internally consistent. That measurement value is translated to different quantities, many based on the chosen Zref (75Ω in this case). Continue reading AIM 914 produces internally inconsistent results

Matched Line Loss of generic RG6/U Quad Shield CCS

This article documents a measurement of Matched Line Loss (MLL) of a 35m test section of generic RG6/U Quad Shield CCS.

It has become impossible in recent years to buy low cost RG6/U with solid centre conductor locally, and the imported product with solid copper conductor is prohibitively expensive (~$6/m as against $0.35/m for the CCS).

The CCS cable does have near copper like performance at UHF and above, but what is its behavior at HF?

Above is calculated MLL from a S11 scan of the test section with S/C and O/C termination. There is a little ripple on the response due to measurement error. The graph also has a curve fit, MLL=0.0285f^0.1506 (F in MHz). Continue reading Matched Line Loss of generic RG6/U Quad Shield CCS

KN5L measurement of JSC 1318 windowed ladder line – MLL @ 4.2MHz

John, KN5L, published some interesting measurements he made of a recent purchase of JSC 1318 windowed ladder line. JSC Wire & Cable is now known as Seminole Wire & Cable, and this is their 1318 product.

Product with apparently similar specifications are sold by many ham retailers, they may or may not be sourced from Seminole.

Some sellers specify the % ICAS rating of the copper clad conductor, usually 30%, some just don’t mention it.

John carefully measured the DC resistance of his line section, and found that it reconciled well with the Copperweld datasheet for 21% CCS.

He also used a VNA to measure S11 of the line section with S/C and O/C terminations, and he gives links to the Touchstone files at the top of his page.

The O/C Touchstone file allows calculation of Zin. The O/C line exhibits resonance at 4.2MHz, at Zin=3.7Ω. His fuller set of measurements showed that Zo at 4.2MHz is very close to 400Ω. We can use those measurements to calculate Matched Line Loss (MLL).

Above, MLL is 0.50852dB/100m.
Continue reading KN5L measurement of JSC 1318 windowed ladder line – MLL @ 4.2MHz