tech stuff – Page 17 – Engineering Radio

The Smith Chart

I have been fooling around with Smith Charts lately. They look complicated, but are really pretty easy to understand and use, once you get around all those lines and numbers and stuff. Smith charts offer a great way to visualize what is going on with a particular antenna or transmission line. They can be very useful for AM antenna broadbanding.

.pdf version available here: smith-chart.

The first thing to understand about a Smith chart is normalization. Impedance and reactance are expressed as ratios of value units like VSWR. A ratio of 1:1 is a perfect match. In the center of the Smith chart is point 1, which expresses a perfect match. To normalize, the load resistance and reactance are divided by the input resistance. Thus, if the input resistance is 50 ohms and the load impedance is 50 ohms j0, then the normalized Smith chart point would be 50/50 or 1. If the load impedance is 85 ohms and the reactance is +j60, then the normalized Smith chart point would be ~~.58~~ 1.7,1.2.

More basic Smitch chart usage information in this video:

I touched on the black art of AM antenna broadbanding before. It is a complex topic, especially where directional antenna systems are concerned, as there are several potential bottlenecks in a directional array. To explain this simply, I will use an example of a single-tower non-directional antenna.

Below is a chart of base impedance from a single tower AM antenna on 1430 KHz. The tower is skirted, 125.6 degrees tall. An AM tower that is expressed in electrical degrees is denoting wavelength. A 1/4-wave tower (typical for AM) is 90 degrees tall. A 1/2 wave tower is 180 degrees tall. Thus this tower is slightly taller than 1/4 wavelength.

Frequnecy(khz)	Reactance	Reactance (normalized)	Resistance	Resistance(normalized)
1390	-j 139	-2.78	405	8.1
1395	-j 143	-2.86	400	8.0
1400	-j 147	-2.94	350	7.0
1405	-j 146	-2.92	310	6.2
1410	-j 142	-2.84	270	5.4
1415	-j 132	-2.64	236	4.72
1420	-j 125	-2.50	210	4.2
1425	-j 118	-2.36	190	3.8
1430	-j 112	-2.24	170	3.4
1435	-j 106	-2.12	155	3.1
1440	-j 100	-2.00	138	2.76
1445	-j 93	-1.86	125	2.5
1450	-j 86	-1.72	114	2.28
1455	-j 79	-1.58	104	2.08
1460	-j 75	-1.50	95	1.9
1465	-j 70	-1.4	92	1.84
1470	-j 65	-1.3	85	1.7

The base impedance is not too far out of line from what is expected for a tower this tall. Plotted on a Smith Chart:

1430 base impedance plotted on a Smith chart

One of the first principles behind broadbanding an AM antenna is to distribute the sideband energy evenly and have a symmetrical VSWR. The antenna tuning unit will match the line impedance to the load impedance and cancel out the reactance. Having the proper phase advance or phase retard rotation will distribute the sideband energy symmetrically about the carrier. To determine phase rotation, the cusp of the plotted graph is rotated to face either the 3 o’clock or 9 o’clock position (0° or 180°). The cusp is where the direction of the line changes, which in this case is the carrier frequency, 1430 KHz. The above example, the line is fairly shallow, which is typical of a skirted tower. Thus, the best phase rotation to start with is +79°. This will likely be close but will need to be tweaked a bit to find the optimum bandwidth. After looking at the plotted Smith chart, my first inclination would be to reduce the rotation, more tower +75° as a first step in tweaking.

When working with AM systems, the bandwidth of the entire system needs to be examined. That means that final bandwidth observations will need to be made at the transmitter output terminal or in some cases, the input to the matching network. It varies on system design, but things like switches, contactors, mating connectors, ATU enclosures, etc can also add VSWR and asymmetry. Broadbanding even a simple one-tower AM antenna can require quite a bit of time and some trial and error.

I will touch on ATU design in the next post.

The Shively 6810 FM antenna

Update, W232AL:

The news is out; this is for the new “WFAS-FM” which is actually W232AL retransmitting the WPLJ HD-2 channel. What do they call translators these days… Metro stations? Something like that. Anyway, quite a bit of work went into getting this off the ground before the start of Labor Day weekend and here it is!

We are currently working on a project that involves installing a Shively 6810 FM antenna. Since few people get to see these things up close, I thought I would post a few pictures.

This particular antenna is a four-bay, half-wave spaced directional antenna. It is going to be side mounted on a 430-foot tower. To do this, we had to lower the AM skirt wires by about fifteen feet and retune the AM antenna.

This Shively antenna came in seventeen boxes with sixty-four pages of assembly instructions. There are many parts and they need to be assembled in the order specified, otherwise, things get in the way. We found that Shively provided many extra bolts, washers, O rings, etc because things get lost. Also, all of the parasitic locations and bay orientations were clearly marked. One thing that the tower crew said; always check the Allen screws and other hardware on the elements before installing the RADOMES.

Since this is a half-wave antenna, the radiating elements are 180 degrees out of phase, bay to bay.

Stainless steel tower leg mounting brackets.

Assembled element with RADOME. This is the top bay with the gas pressure release valve

We hoisted two bays at a time. The top bays are ready to go up.

The bottom two bays were hoisted next.

This is the antenna installed, less the tuning section and parasitic elements. It is tilted off axis from the tower by design due to its highly directional nature.

The transmission line was installed and we swept the antenna. I will snap a few final pictures once the transmitter is installed, which will happen tomorrow.

Updated Pictures: Here are a few pictures of the finalized installation:

W232AL antenna, new installation on WFAS AM tower

The fully installed antenna, tuning unit and transmission line. We did some program testing, made sure the audio sounded good, then the station was signed on. We also had to lower the AM station’s skirt a few feet and retune the ATU. Actually, the ATU needed to be reconfigured because the shut leg had been disconnected and there was a capacitor added to the circuit after the base current meter. All of that was fixed, along with a few other things…

W232AL transmitter, a BW Broadcast TX300 V2

The W232AL transmitter is a BW Broadcast TX300 V2. These little transmitter are packed with features like a web interface, on board audio processing, etc. They are pretty neat.

Its Friday, time to go home! — It’s Friday, time to go home!

The tower crew from Northeast Towers did a great job, as they always do.

Transmitter repair

Sometimes it is obvious and relatively easy, other times not so much. This summer we have had wave after wave of afternoon thunderstorms. It is almost like living in Florida; almost, but not quite. Anyway, with the storms occasionally comes some lightning damage. At most of the transmitter sites we service, every step has been taken to ensure good grounding and adequate surge suppression. This is especially true of sites that have been under our care for a few years. Even so, occasionally, something gets through. After all, those five-hundred-foot steel towers do attract lightning.

Broadcast Electronics AM5E output tuning section

This is the output section of the BE AM5E transmitter at WROW. The transmitter got pretty trashed; a bad PA module and power supply and this capacitor in the output section. This particular transmitter is 14 years old and this is the first major repair work we’ve had to do it.

Broadcast Electronics AM5E output tuning capacitor

The capacitor was fairly easy to change out. As a general precaution, both capacitors were changed. There was a spare PA module and power supply on the shelf, thus the transmitter was returned to full power relatively quickly.

Broadcast Electronics AM5E output forward and reflected power meters

The rest of the antenna system and phasor were inspected for damage, a set of common point impedance measurements taken, which showed that no other damage was sustained.

Next, the 30 year old Harris SX2.5 A transmitter at WSBS. This failure was slightly more exotic; the transmitter started randomly turning itself off. The culprit, in that case, was this:

Harris SX2.5 remote control interface bypass capacitor

Literally, a two cent part. The transmitter remote control uses optoisolators. The inputs to these opto-isolators are RF bypassed to ground on the back of the “customer interface board.” After determining that the remote control was not malfunctioning, it was down to either a bad opto-isolator or something really silly like a bypass capacitor. This capacitor was on the ground side of the remote off terminal. It shows short on the capacitance meter and 4.1 K on the ohm meter, just enough to randomly turn the opto-isolator on and shut down the transmitter. Being a Harris transmitter, removing and replacing the “customer interface board” was no easy matter. Overall, it took about three hours to find and repair this problem.

The rotary phase maker

I alluded to this in an earlier post: Open Delta three phase service. Some transmitter sites are fairly remote and three-phase power is not available. Occasionally, with lower-powered radio stations, this is acceptable because those transmitters can be configured to run on single-phase power. However, almost any transmitter above five kilowatts or so will require three-phase power. This is the case at the WQBJ transmitter site in Palatine Bridge, NY. The site is located in the middle of farmland and only has single-phase service. The nearest three-phase service is several miles away and the utility company wants several hundred thousand dollars to upgrade the line.

WQBJ transmitter site electrical service

The station is a class B FM with a six-bay full wave-spaced antenna. Even so, the TPO is 17 KW, which makes some type of three-phase service a requirement.

WQBJ six bay Shively 6810 antenna — WQBJ six-bay Shively 6810 antenna

The main transmitter is a Broadcast Electronics FM30B, which is now 25 years old.

WQBJ main transmitter, Broadcast Electronics FM30B

The backup transmitter is a CSI FM20T, which is almost forty years old.

Rather than do an open delta service, which is not desirable for several reasons, both transmitters have their own rotary phase makers. From a reliability and redundancy standpoint, this is the right way to equip this site. The rotary phase makers are essentially a motor generator combination which takes the split phase power and generates a third phase.

WQBJ phasemaster, backup three phase converter — WQBJ Phasemaster type T, backup three-phase converter

The phasemaster is is a 40 KVA unit and is connected to the backup CSI transmitter

WQBJ Roto Phase, main three phase rotary converter — WQBJ ARCO Roto Phase, main three-phase rotary converter

The Roto Phase unit for the main transmitter is actually two 40 KVA units connected in parallel through dry core isolation transformers. Incidentally, the Roto Phase units need to have their bearings changed every ten years or so. This requires the units be disconnected, placed up on their end. To get the old bearing out, the housing has to be cooled with liquid CO2. Both units are due for new bearings soon, which should be a pleasant job indeed.