Entries from January 2013 ↓

Quad Construction

One of the more troublesome aspects of quad construction is providing an effective support of the arms that hold the quad loops. Then there is also the problem of securing the quad loops to a suitable boom. Not that anyone would try to use an unsuitable boom but some have tried using unsuitable means of attachment. We are defining unsuitable as any method that does not allow the finished quad to survive at least one year of seasonal weather.

Consider the following.

Spiders to hold support arms can be made from short lengths of square metal tubing. These tubing pieces can be welded together at right angles. Additional strips of metal can be welded at right angles to these square tubing assemblies and used to secure the spiders to a boom. If metal booms are selected, the strips of metal serving as boom mounting tabs can be welded to the metal boom or the tabs can be drilled and mounted using sheet metal screws or more conventional nuts and bolts.

Use of aluminum or stainless steel is preferred but mild steel may be more readily available and weldable. A mild steel assembly can be protected by painting. A good brand of paint like Rustoleum is effective and economical with a single can of spray paint being enough to protect several dozen spiders.

The square tubing spiders are hollow all the way through allowing hardwood dowel ( or special hardwood square strips cut on a table saw ) to be used as spreaders to support the wire elements. The hardwood spreaders can be painted or boiled in paraffin to protect them from the weather. Or use plastic rods. The supports can be held in place in the metal spiders with screws driven through a tapped hole in the spiders or they can be glued in.

Practical dimensions can be relatively small for VHF quads. A two meter version could easily be implemented using 1/4 inch square tubing lengths no longer than 1 inch. Spreader arms would be less than three feet.

This method of construction can be scaled up for HF frequencies. Spiders made from 1 inch square tubing as short as 3 inches in length might be suitable for a multiband quad with 20 meters being the lowest band covered.

I would prefer to use square tubing because it is easier to make a square weldment using square tubing but there is no reason round pipe could not be used as well. In my case I would also select a square boom either metal or wood. The only construction material I would exclude is PVC pipe. I have seen too many projects that use PVC pipe and the only ones that look presentable are the collinear vertical arrays where the pipe only serves as a radome containing the antenna. All other applications have a distinctly tacky plumbers look to them.

Unreasonable Antenna Claims

Recent claims of 10dBi gain with 18 dB front to back for a shortened coil loaded mini two element 20 meter beam were both unreasonable and unbelievable.

Many publishers are quick to divorce themselves from what they publish by claiming that the stories they print do not reflect their views yet they still print them. Okay, maybe doing so sells magazines, at least for one issue. Unfortunately it also turns off the thinking reader to the point of avoiding future issues.

A gain of 10 dBi? You can always bet on inflated gain claims when dBi is invoked. The dBi designation is a phony baloney scale based on how an antenna compares to an isotropic antenna. There is no such thing as an isotropic antenna in the real world. An isotropic radiator is a theoretical point source radiator. Even an ordinary dipole has about 2.5 dBi gain! What could possibly cause people to make comparisons to things that don’t exist? Could it be the money they seek by misrepresenting their product and defrauding potential buyers?

So far we have accounted for 2.5 dBi out of the 10 dBi claimed. Lets see if we can find the 7.5 dBi remaining.

A two element beam is most commonly composed of a driven element and a reflector spaced 1/4 wavelength behind the driven element. The reflector is energized by that half of the signal radiated by the driven element in the undesired direction. Once energized, the reflector re-radiates the signal it has intercepted. That 1/4 wave separation between driven element is important because it ensures radiation will travel 1/2 wavelength (from driven element to reflector and back to driven element) to ensure it is in phase with the signal being radiate by the driven element so it can be added to the radiation at the driven element.

Under the best of circumstances this mechanism can only double the effective radiated power for a gain of 3 dB. Now add the 2.5 dBi we get from a dipole apply some real world inefficiencies, and we arrive at a 5 dBi gain for a full sized two element beam. That is the historically accepted gain figure for this antenna.

Only 3 dB! We can do that by simply doubling the transmitted power. True but increasing transmitter power does nothing for the received signal. Three dB at the antenna doubles both transmitted and received signals.

The article in question now claims that by shortening and reducing element diameter, adding loading coils, and reducing the 1/4 wavelength separation between elements, we can double the gain to 10 dBi!!!

I believe you would have better luck sprinkling fairy dust on that isotropic radiator.

All Band QRP Rigs

I see quite few of these jewels on the used market. Previous owners are asking about 80% new price which tends to discourage buyers. Why would I pay up to 80% of the new price of someones discarded play toy? Why not buy new for just 20% more and get a new rig warranty along with real mint condition fresh from the factory?

Perhaps a better question is, ‘Why pay almost $1000 for a transceiver and accessories that can’t guarantee reliable communications’?

I admit that at times QRP might be fun to play with but the fun is not there when the QRP rig costs more than $200. For $1000 you can get a real radio that you can use for serious communications.

Evidently these would be sellers of used QRP rigs have recognized this fact and are hoping to recoup as much of their bad investment as possible.

Unfortunately most prospective buyers are not interested in paying the freight on a strangers silly mistake. Most used radios are seldom worth more than half their new purchase price regardless of age or condition.

Why Windom

I wanted an all band antenna. That is why I chose a Windom. It is pretty much all band but works best on 40,15,12, and the upper part of 10 meters. I have a beam for 10 and 15 and never get on 12 meters so I end up using the windom on 40 where a dedicated 40 meter antenna might be better.

So I am modifying my Windom into a 40 meter loop. Still keeping the 4:1 balun as long as the SWR remains decent. The present feed point is close enough to a vertically polarized delta loop and the impedance should be close to 150 ohms. That comes out to about 2:1 for 75 ohm coax and 1.3:1 for 50 ohm coax. We will just have to try it and see what we get.

Half wave for 3.5 mhz is full wave for 7.0 mhz. It should work out pretty close and this will be the first time I have a 40 meter delta loop with top apex at 60 feet.