Review of N6PAA’s 40m STL

(N6PAA nd) describes several small transmitting loops (STL) and gives some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

This review focusses on his 40m STL.

The loop is a circle of perimeter 3.83m which at 7.1MHz is 0.091λ which is at the top end of the strictest criteria for an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops). It appears from his pics that the bottom of the loop is about 1.5m above real ground, so we expect a significant ground loss resistance component in Rtotal.

N6PAA gives a measured VSWR curve for the matched antenna, and the VSWR=3 bandwidth as scaled from the graph as 20kHz, from which we can calculate the half power bandwidth and eventually, efficiency. There is some suggestion that some measurements were taken indoors, this analysis assumes that the relevant measurements were taken outdoors as pictured. Continue reading Review of N6PAA’s 40m STL

Review of KK5JY’s 40m STL

(Roberts 2010) describes several small transmitting loop (STL) and gives some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

This review focusses on his 40m STL.

The loop is a circle of perimeter 4.3m which at 7.1MHz is 0.102λ which is at the top end of the strictest criteria for an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops). It appears from his pics that the bottom of the loop is about 2m above real ground, so we expect a significant ground loss resistance component in Rtotal.

Roberts gives the VSWR=2 bandwidth as 5.4kHz, which if we assume that it was adjusted for a perfect match mid band, we can calculate the half power bandwidth and eventually, efficiency. Continue reading Review of KK5JY’s 40m STL

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #4

Fourth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a weep hole would be advisable in a permanent outdoor installation).

AtuBalun201

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated.

AtuBalun203

The interior shows the layout. The wires use XLPE high temperature, high voltage withstand, moderate RF loss insulation. Two short pieces of 25mm electrical conduit serve to position the balun core against the opposite side of the box, and a piece of resilent packing between lid and core holds the assembly in place.

AtuBalun202

Differently to the example shown in the earlier articles, this prototype uses twisted PTFE insulated wires which have voltage breakdown higher than the XLPE shown earlier.

Clip 124

The self resonant frequency of the built balun was measured as 7.4MHz and the predictive model above calibrated. The balun has high choking impedance on the lower half of HF.

End fed matching – PA3HHO design review #2

 

A correspondent having read End fed matching – design review raised a similar design by PA3HHO which uses a#43 ferrite toroid as part of an end-fed matcher, see Multi band end-fed (English).

Further to End fed matching – PA3HHO design review , this article has a solution to PA3HHO’s “original 40/20/10 version” using FT140-43 with a 2t primary.

Continue reading End fed matching – PA3HHO design review #2

Reconciling my #52 choke design tool with G3TXQ’s measurements

A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.

FT240-52x2-11t

The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

Screenshot - 09_02_16 , 18_29_42

To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale). Continue reading Reconciling my #52 choke design tool with G3TXQ’s measurements

Exploiting your antenna analyser #14

Insertion Loss, Mismatch Loss, Transmission Loss

A correspondent asks about the effect of RCA connectors at HF on his proposed noise bridge. The question is very similar to that considered at Exploiting your antenna analyser #13 for UHF series connectors.

I have made a simple measurement of a BNC 50Ω termination (to check calibration) then inserted a BNC-RCA and RCA-BNC adapter.

Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.

To jump to the chase, it also has a Smith chart plot up to 200MHz that suggests it might be well modelled by a TL segment of 30-35Ω.

Screenshot - 07_02_16 , 16_58_55

Above is a plot of VSWR when Zref is adjusted for the flattest response from DC, and it can be seen that with Zref=33, response is quite flat to 200MHz. Continue reading Exploiting your antenna analyser #14

Exploiting your antenna analyser #13

Insertion Loss, Mismatch Loss, Transmission Loss

A correspondent having read Exploiting your antenna analyser #12 asks whether the measurement provides evidence of loss of the connectors, and referred me to (Arther nd) where he reports some measurements of UHF series adapters and conclusions.

Duffy

Let’s deal with interpretations of my own measurements first.

Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.

Screenshot - 01_02_16 , 11_40_57

Above is a plot of ρ and ReturnLoss for the DUT. ReturnLoss curiously is plotted ‘upside down’ as ReturnLoss increases downwards… a quirk of AIM software, but remember that ReturnLoss in dB is +ve.
Continue reading Exploiting your antenna analyser #13

Low noise Yagis and 50MHz noise

After reading my post Some thoughts on noise on 6m (50MHz)…, a correspondent asked about the content in the context of comments by G0KSC.

Under the altruistic heading “Low Noise Yagis Explained”, G0KSC takes a competitor to task over the accuracy of statements regarding the importance or not of G/T at 50MHz.

G0KSC says:

MISREPRESENTATION OF PERFORMANCE MEASUREMENTS SUCH AS G/T:

Have you ever heard the saying ‘A little knowledge is a dangerous thing’?…

What is G/T? Continue reading Low noise Yagis and 50MHz noise

Voltage and current on a transmission line with standing waves

Folk often ask how to calculate the maximum voltage on an antenna feed line with standing waves, often to get a feel for the necessary voltage withstand of baluns, feed line, switches and relays, and ATUs.

Feeding at a current maximum outlines the method described in detail at (Duffy 2011), but the approach is more complex than a lot of hams want.

A simpler method is to treat the transmission line as lossless, and to simply find the worst case voltage and current that can occur… and design for that, or perhaps do the more detailed analysis depending on the outcome.

A new calculator, Calculate Vmax, Vmin, Imax, Imin for lossless line from Zload (or Yload) and Zo, does just that.

Screenshot - 29_01_16 , 19_45_17

Above is the built-in example of a G5RV with tuned feeder on 80m with feed point impedance derived from a modelling package. The voltage and currents calculated are those for a long lossless feed line.
Continue reading Voltage and current on a transmission line with standing waves