## MFJ-1786 loop antenna – a study of the matching scheme

The article MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz observed of the plot of loop impedance: It looks quite different to the expected behavior of the underlying loop, but it does contain an arc albeit rotated and offset. In fact it can be transformed in two simple steps.

## MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz – analysis tools

Further to MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz, the question arises as to what commonly used tools readily permit the transformations and analysis.

Some relveant theory: for a load where R is approximately constant and X varies, the half power points occur where R=|X|, and following on from that s11=0.2±j0.4, s11=0.4472∠63.43°, |s11|=-6.99dB, ReturnLoss=6.99dB (yes, the +ve sign is correct), VSWR=2.618 etc. Finding the points where ReturnLoss is approximately 6.99dB with the cursor on the above diagram is quite easy. Continue reading MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz – analysis tools

## MFJ-1786 loop antenna – other models at 10.1MHz

Further to MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz this article presents some other models of expected performance of the MFJ-1786 loop.

## AA5TB model

One of, if not the most popular loop calculator cited by hams is that by AA5TB. It is especially praised by ham loop enthusiasts. Above is a screenshot of AA5TB’s calculator with the real antenna dimensions and “Added Loss Resistance” to calibrate the model to the measured 8kHz half power bandwidth. It predicts an efficiency of 30.6%, 2.9 times that of the NEC model. Perhaps it is popular because it provides overly generous estimates, IMHO it lacks credibility for many reasons. Continue reading MFJ-1786 loop antenna – other models at 10.1MHz

## MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz

My friend Carlos, VK1EA, made some measurements of an MFJ-1786 SUPER HI-Q 36″ (0.914m) DIA 10-30 MHz loop at 10.1MHz.

This article presents some modelling and analysis of the antenna principally to estimate its performance.

The loop was located at 2m above natural ground away from other conducting objects.

He tuned the loop for minimum VSWR at around 10.1MHz and took a sweep with a EU1KY antenna analyser looking through 0.5m of RG223 50Ω cable saving the results to a s1p file which was imported to Antscope.

## Measurement of the real antenna

Here is the impedance plot (excuse the |Z| plot as it is Rigexpert’s concession to hams who do not understand impedance and I cannot disable it). Above, the impedance plot. The cursor is at point of minimum VSWR, and the associated R and X values at the measurement point are not very useful. Continue reading MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz

## NEC model of figure 8 transmitting loop

Variants of loops have been designed and promoted as having certain advantages, and one of those is the so-called figure 8 loop.

This article describes an NEC-4.2 model at 14MHz of an antenna similar to a commercial example. The graphic shows the geometry. In this case the source is at the bottom of the lower loop, and the blue square is the tuning capacitor. The loop conductor is 22mm copper tube, the loop diameters are 1m, and the capacitor connection is 100mm wide. Commonly these are fed by a low loss auxiliary loop at the bottom of the lower loop, but the direct feed is quite fine for modelling the loop performance. Continue reading NEC model of figure 8 transmitting loop

## W5KV’s transmitting loop measurements – DELUXE HG-1 PreciseLOOP 7MHz

Assuming the measurements were made with the antenna clear of disturbing conductors etc, in good condition. Above is his VSWR scan.

The key measurements were:

• centre frequency 7.175MHz, VSWRmin=1.1;
• VSWR=3 bandwidth 36kHz.

Based on that, we can estimate the half power bandwidth to be 30kHz if R is less than Ro, more like 33kHz in the other case, but we will be optimists.

A NEC-4.2 model of the antenna at 14MHz was built and calibrated to the implied half power bandwidth (30kHz). Model assumptions include:

• ‘average’ ground (σ=0.005, εr=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 W5KV’s transmitting loop measurements – DELUXE HG-1 PreciseLOOP 7MHz

## 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 (σ=0.005, εr=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 (σ=0.005, εr=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