K3EUI’s MyAntennas EFHW on 80m

Barry, K3EUI, posted some interesting measurements of his MyAntennas EFHW which he described with some useful detail:

I have been experimenting with a “MYANTENNA” 130 foot long “end-fed” with the 49:1 UNUN*

I replaced their 130 ft antenna wire with a heavier gauge #12 stranded insulated wire (I had a few hundred feet).*

This is classified as a ONE-HALF wavelength antenna on 80m, hence the need for the 49:1 UNUN to transform 50 ohm (coax) to a few thousand ohms*

It has resonances on the other ham bands (harmonically related) but I wanted it mostly for 80m.

One leg runs about 60 ft horizontally to a tree and then the next 70 ft makes a 90 degree bend (to fit into my yard) still horizontal.*

At this time I removed a 15 ft “counterpoise” wire on the GND side of the UNUN.

I will try it later this week (after the snow) as a “sloper” or an Inverted V up to a tall fir tree.

It is only about 20 ft above ground now (with 4 inches snow) for NVIS prop, and fed with 70 ft of RG213 coax (50 ohm) with a RF choke on the coax feed line 10 ft from UNUN (the counterpoise?) and another RF choke just as the coax enters the shack.

His VSWR curve is interesting, a minimum at source end of about 1.32 @ 3.66MHz as built and measured.

Minimum VSWR is about 1.32 @ 3.66MHz. Continue reading K3EUI’s MyAntennas EFHW on 80m

Last update: 14th April, 2024, 1:34 PM

The black art of common mode current and two wire transmission lines

One of the very popular topics on ham social media is common mode current, and it seems whilst opinions are presented as fact, there is little sound science in evidence.

In a two wire transmission line, we can get good insight into the state of current balance by measuring three currents at a point along a transmission line:

  • I1 in one conductor;
  • I2 in the other conductor; and
  • I12 being the sum of the currents.

These can be measured using an RF current probe, essentially a current transformer for RF, and in the case of I12, it is measured by placing BOTH conductors through the centre of the current transformer so the flux is due to I1+I2 (not simply |I1|+|I2|). There are other ways to obtain I12, but in concept they work the same as passing both conductors through one current transformer. Continue reading The black art of common mode current and two wire transmission lines

Last update: 19th February, 2024, 3:42 AM

Paralleling two synchronous AC generators – simple AVR discussion

Paralleling two synchronous AC generators explained synchronisation and parallel operation of two generators, discussing sharing of real and reactive power.

In that discussion, the engines were speed governed, but voltage was manually controlled by adjusting excitation current. The phasor diagram showed that there is a relatively large source impedance (dominated by the synchronous reactance), EMF is around 1.5 times Vt, and terminal voltage Vt will vary widely with changes in load as a consequence. Continue reading Paralleling two synchronous AC generators – simple AVR discussion

Last update: 17th February, 2024, 10:15 PM

Youtuber on “The myth of SWR”

A Youtuber recently published some enlightenment entitled “The myth of SWR”.

He is obviously a disciple of the late Walt Maxwell and his re-re-reflection explanation:

Here’s a sample of what’s coming which may make your head explode. This is from antenna engineer Walt Maxwell, W2DU.

Now this video has been published just two weeks, it has:

  • 13,693 views in two weeks;
  • 549 likes;
  • 228 comments mostly positive, but a few voices of reason.

Continue reading Youtuber on “The myth of SWR”

Last update: 16th February, 2024, 11:38 AM

Paralleling two synchronous AC generators

This article discusses adding a second synchronous AC generator in parallel with a running synchronous AC generator.

For explanation, a simple context is used, two identical diesel engine driven generators with governed speed, no droop compensation, otherwise largely manual control, simple equivalent circuit of the machines. The complexity of other factors like  harmonic currents etc are not discussed.

The simple model is useful as it underlies practical machines and is a good configuration to introduce the main principles of the operation.

To avoid typos, some of the text uses Python formatting of complex numbers.

Prerequisites for paralleling three phase synchronous generators

Prerequisites for paralleling three phase machines are:

  1. same phase sequence (eg both A-B-C);
  2. nearly the same voltage; and
  3. nearly the same frequency, and eventually same phase.

(1) does not apply to a single phase machine. (1) is resolved at the time of wiring up the machines and circuit breakers used to connect them to the bus. Unless changes have been made to machines or wiring, sequence should not need to be checked. After any reconfiguration or wiring changes phase sequence should be checked by a competent person.

Phase sequence is checked using an instrument like one of these above. The left one is a small three phase motor, the right one is electronic and shows sequence with an LED. Continue reading Paralleling two synchronous AC generators

Last update: 24th February, 2024, 10:47 AM

Youtube – measuring velocity factor of coax cable

I keep being offered Youtube videos showing how to measure velocity factor of coaxial line.

I did indulge one this afternoon. The author explains that measuring s21 phase is the basis of his measurement.

The DUT for the demonstration is 3.76m of coax, no mention of where it was measured from and to.

No mention of the calibration details, so we might assume that a short jumper was used to connect Port 1 to Port 2 for the through test, perhaps the very one shown in the pic below.

Above is the test jig, one end of the coax (UHF plug) attaches to a UHF-F to SMA-M adapter which is attached to the VNA. The other end of the coax appears to connect via a UHF-M to a UHF-F to SMA-F adapter, and the 100mm long jumper cable. Continue reading Youtube – measuring velocity factor of coax cable

Last update: 11th February, 2024, 8:01 PM

Synchronous generator – phasor diagram and explanation

This article offers a phasor diagram for one phase of a synchronous generator and explains the diagram.

System characteristics:

  • Load: vt=240, s=20000VAr; pf=0.85; and
  • Generator (s): s=20000VAr; pf=0.9; ra=0.04pu; xs=0.7pu.
    .

Above is the phasor diagram for generator #1 in a pair of parallel generators. Green is flux, cyan is armature current, magenta is voltage. Continue reading Synchronous generator – phasor diagram and explanation

Last update: 11th February, 2024, 9:59 AM

Synchronous machine power formula

An expression found in most texts on synchronous machines relates power to the power angle δ. δ is the angle between the terminal voltage and the generated EMF (under the applicable load current).

\(P=\frac{|E|\cdot|V|}{X_s}sin \delta\\\)

V and E are RMS values, albeit sometimes with phase.

This article gives a derivation that exposes the underlying assumptions that are not usually mentioned.

 

Above is the phasor diagram of the machine in generator mode. Continue reading Synchronous machine power formula

Last update: 11th February, 2024, 6:52 AM