Entries from January 2015 ↓

Moxon Configuration

How critical is the physical configuration of a Moxon rectangle? How rectangular does it have to be?

I can’t give actual tolerances or even measurable before and after results but I can relate a recent experience that suggests minor deviations from true rectangular geometry are allowed.

I built a wire Moxon for 40 meters according to the formula and supported it on two trees, the chimney mount and one eave of the house. The height at the trees was about 30 feet, The chimney at 20 feet, and the eave at 15 feet. The orientation was SE to NW with the antenna favoring NE and SW directions. The parasitic element was rigged with a relay system that could make it a reflector or director.

It seemed to work. On several occasions signals were effected up to 15 db on the S-Meter by flipping from reflector to director.

It was less than perfect. One end was too low and I suspected I had lots of feedline radiation when running high power. High power operation caused interference with computers, radios, tvs, and dvd players. Most of these problems were solved by adding an rf choke to the antenna feedpoint but I also wanted to raise the Moxon to at least 30 feet. I could do that if I suspended it from the trees and two towers. Trouble was that the distance between the main mast and the closest tree was only 42 feet. The stock moxon rectangle needed 50.5 feet.

My first attempt at getting to 42 feet was to take it out of the middle, at the feedline, by dropping the feedpoint down by 4 feet. I ended up making sort of a modified delta feed with the top of the delta measuring about two feet. The two foot distance was maintained with two feet of parachute cord and PVC insulators.

This allowed the long section of the rectangle to fit into the 42 feet available but it also raised the resonant frequency by 300 khz. I tried winding the 4 foot wire drops at the feedpoint into coils to lower the frequency. For reasons I still do not understand, that did not work. The resonant frequency remained 300 khz too high.

After sleeping on it, the next day I decided to take the 8 feet out of the long dimension by moving the support points 4 feet closer in to the feedpoint. I did the same for the parasitic element. After this wire antenna was raised to the 30 foot level, the resonant frequency came back down and the antenna exhibited a broad band characteristic. The 1:1 SWR points were measured to be from 7.140 to 7.340. the SWR at 7.000 was only 1.5:1. A little tweaking would get the 1:1 SWR points into the middle of the band but this was close enough to be useful.

Initial testing verified that the wire beam still exhibited some directional characteristics. Appears that it has a 20 db front to back. It is now oriented to beam east or west. Its physical dimension is now 42 by 25 feet. The original configuration had it 50 by 17 feet. This 8 foot change in dimensions did not appear to have a significant effect on performance. Note that the total lengths of the wires were not changed. They were merely re-distributed to change the long dimension from 50 feet to 42 feet.

SWR and Power Meter Accuracy

Is a high degree of accuracy necessary when measuring SWR and power? It depends on what you intend to do with the measurement.

Most of us just need to adjust our antennas for minimum SWR. As long as that minimum can be determined to be less than 1.5:1 we have met our goal. It does not matter if that value is 1:1 or 1.2:1.

Likewise, most of us just tune our amps for maximum output. Knowing the exact value of that output does not make the amp work any better. A relative indication of power out is more than adequate for most uses.

How to save $1000 on Ham Radio Equipment

You can save at least that much by following consumer purchase rule number one. Don’t spend money or time on stuff you don’t need. I guess we could debate the need for any radio equipment. We could live without radios but we have already decided to live with them so there is no need to consider such debate.

There are three fairly expensive devices that most hams covet that are really not necessary. These include, a watt/SWR meter, an antenna analyzer, and an antenna tuner.

Most modern radios offer built-in automatic antenna tuners. Manufacturers started doing thus to keep their solid state SWR protected transmitters delivering power. Without these tuners the solid state rigs will start folding back on power output with SWR levels as low as 2:1.

With the built-in tuners high SWR levels are no longer an issue until we upgrade our stations with a power amplifier. Suddenly we find ourselves in the market for a high power/high dollar tuner to keep the new amp from melting down. We can end up spending big bucks treating the symptom of a problem that can easily be avoided by choosing a resonant antenna system at little or no cost.

As a bonus we can completely avoid concerns regarding tuner efficiency and feedline losses by choosing an antenna system that offers us less than a 1.5:1 SWR. Try it. You might be able to hear better too.

The second unnecessary item is the antenna analyzer. Most modern transceivers can do all the antenna analyzing necessary. The transceivers have built-in SWR meters and are far more accurate and stable than most antenna analyzers. Okay, a good analyzer will also tell you what impedance the antenna is reflecting back to the transmitter. Do you really need to know that to trim your antenna to a lower SWR?

The third Item is a good SWR/Power meter. A really good device can cost as much as $1000 all by itself. It will give you an accurate indication of power output but you can also get that by reading the plate voltage and plate current off the meters in your amp without having to spend additional money.

Keep in mind that knowing how much power the rig is delivering to the transmission line is not going to allow you to make more contacts.

Also, that high dollar antenna tuner might make your powerful amp happy but it is not going to keep all that happy power from frying your 300 ohm twin lead if the SWR is high on the feedline.

Antenna Tuners

If your antenna tuner is in a box sitting on your operating desk it is more of an impedance matching network that a tuner. Impedance matching is a good thing because it allows your transmitter to develop maximum power.

However, if your transmission line is long, lossy and has a high SWR, that power might be dissipated in heat before it gets to the antenna. Also, under those conditions, power ratings of the tuner become inconsequential because the impedance mismatch on the antenna side of the ‘tuner’ may very possibly have increased the voltage across the output capacitor to a value sure to arc and destroy the plates.

So what is the solution?

Don’t use antenna tuners. If all your antennas are resonant you don’t need tuners.

SWR and Power Meters

Is it worthwhile to pay extra for a power meter that can measure up to 2000 watts and beyond?

If we want to measure and monitor the output of a powerful amplifier we would need such a device.

Perhaps a better question would be, ‘Why do we want to measure the output of our amplifier’?

Have we received reports from long time ham friends that our signal has become weaker? Maybe we just want to check and make sure our tubes are not soft. Whatever the reason, an accurate measurement of the amplifiers output is not going to improve the signal. In addition, a truley accurate, high power, watt meter is going to be expensive. You would be better off spending that money on new tubes if you suspect the old ones to be soft. Besides, you can get a pretty good idea of the power level by noting the plate current and voltage under load. The product of plate current and plate voltage times .6 is a good indication of power out.

Tubes are not the only thing that can go soft. Plate voltage can droop due to aging electrolytics in the plate supply. A lower plate voltage will result in lower plate current as well.

You may also want to make sure you are not expecting more output than your amp can deliver. Don’t confuse output with input. Most amp manufacturers prefer to quote power input and list it in PEP. PEP is normally about twice what you can expect from a key down CW condition and to be linear an amp can only deliver about 60% of the input power at its output. That means a 600 watt PEP input amp is only capable of delivering about 200 watts constant carrier key down output.

Input only sets the cost of running the amp. Output is what is useful to the amateur radio operator. The benefits of knowing precisely what that output is do not justify the cost. So, no, you should not pay extra for a high powered watt meter.

Getting ahead of myself, I will also state that you don’t need a high power SWR meter either. A low powered simple resistive bridge can be built with very short lead lengths allowing its use up through the UHF region.

Also you don’t need a dual meter dual pot sensitivity adjustment. A single meter with sensitivity pot in the return lead of the meter will work just fine if it is switched from the forward circuit to the reflected. First, reading forward, you adjust for full deflection. The you switch to read reflected. If the reflected reading is at half scale the SWR is 3:1.

I already have a decent SWR meter, but, if I were looking for one today, I would buy the least expensive CB meter I could find, gut it, and install a resistive bridge for a pickup. I have seen prices as low as $15 on Amazon. That is less than you would have to pay for a meter, switch and case.