Friday, October 29, 2010

What is wrong with operating FM simplex between 145.8 and 146 MHz? ...

Q What is wrong with operating FM simplex between 145.8 and 146 MHz? I hardly ever hear signals in that part of the band.

A This touches on the issue of voluntary band planning, which was addressed in “Washington Mailbox” in the December 2000 QST. You might want to take a look at that column.

The problem with operating in the 145.8 to 146 MHz segment is that the amateur satellite community uses this portion of the spectrum. There are several FM repeater satellites that operate with uplinks in this segment. If you’re chatting with a local friend on 145.85 MHz, for example, there is a chance that OSCAR 27 will hear you and relay your conversation over thousands of miles without you even realizing it! The FM satellites are crowded already, so they don’t need “unintentional signals.” Other SSB and digital satellites have uplinks here as well, and your signals could make it impossible for these birds to hear the signals intended for them.

SO-35 and AO-10 have downlinks in this portion of the band. These signals are often weak and your terrestrial FM QSOs will obliterate them.

Just because you can’t hear the satellite signals, doesn’t mean that they are not there. With the typical FM setup, you’re not likely to hear satellites in this portion of 2 meters at all.


Voluntary band planning allows everyone to enjoy Amateur Radio in all of its various forms, but it only works if we all respect the plans. It isn’t so much a legal issue as it is one of common courtesy. 


From QST February 2001

Every time you double your antennas, say, going from a single VHF/UHF Yagi to a stacked pair of Yagis ...

Q Every time you double your antennas, say, going from a single VHF/UHF Yagi to a stacked pair of Yagis, you realize an increase in gain, but you also lose a certain amount of power through the power divider or phasing harness. How do you manage to come out ahead?

A It is true that stacking antennas produces additional directivity and gain. You can stack antennas vertically (Figure 1) or horizontally, although vertical stacking is most common. 

The “secret” is in the fact that gain can only come from taking power that would otherwise be radiated in other direction(s) and concentrating that power into the main, desired lobe(s).

The most easily understood physical demonstration is one that I’ve used for years at radio club meetings (see Figure 2). I take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, I blow up another balloon to the same size and shape and tell the audience that this will be my “reference” antenna.


Then I squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that I’ve now created a dipole and compare the maximum size to that of my reference “antenna.” The dipole can be seen to have some “gain” over the reference isotropic. Next, I squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna would have, again comparing the gain to the reference isotropic antenna, er, balloon.


By combining antennas in a stack, you can accentuate this gain and directivity even further. In the end, you have created much more total gain in the antenna system than would be lost in the power dividers or phasing harnesses. Stacking isn’t easy or inexpensive, but the performance gain can be substantial.

Figure 1—An example of vertical antenna stacking. 


 Figure 2—Demonstrating antenna pattern gain with balloons. Take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, blow up another balloon to the same size and shape and tell the audience that this will be the “reference” antenna (A). Then, squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that this is a dipole and compare the maximum size to that of the reference “antenna” (B). The dipole can be seen to have some “gain” over the reference isotropic. Next, squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna creates (C).

From QST February 2001

Thursday, October 28, 2010

When it comes to HF antennas, how important is the elevation angle?...

Q When it comes to HF antennas, how important is the
elevation angle?

A Presuming that you are interested in working worldwide DX on the HF bands, the vertical (elevation) angle of maximum radiation is of considerable importance. An elevation angle of 5° is very shallow, while 90° is straight up (not a good angle for long-distance communication!). You want your radiation pattern to be at a low elevation angle so that the signal energy will be refracted by the ionosphere in such a way that it propagates as far as possible (see Figure 1).

Tables 1, 2 and 3 from The ARRL Handbook show optimum elevation angles from locations in the continental US. These figures are based on statistical averages over all portions of the solar sunspot cycle.


Since low angles usually are most effective, this generally means that horizontal antennas should be high—higher is usually better.










Figure 1—The elevation angle advantage. If your signal takes off at a high elevation angle (A), it won’t propagate very far. Lower the angle (B), and the increase in distance can be considerable. A wavelength of height at a particular frequency results in a peak elevation angle of about 15°. 































From QST January 2001
 

I recently bought some radio crystals. Most are removed from 1940s Navy radios ...

Q Jon, W4BCT, asks, “I recently bought some radio crystals.
Most are removed from 1940s Navy radios. When I
was young my father had some of these, and I wanted to take them apart to see what was inside. Of course, he wouldn’t allow this. Now I have some to play with, but I was wondering if you could explain how crystals work?”


A A number of crystalline substances found in nature have the ability to transform mechanical strain (movement) into an electrical charge, and vice versa (think of a tuning fork or a church bell which can transform mechanical strain into sound). This property is known as the piezoelectric effect. A small plateor bar cut in the proper way from a quartz crystal and placed between two conducting electrodes will be mechanically strained when the electrodes are connected to a source of voltage. Conversely, if the crystal is squeezed between two electrodes a voltage will be developed between the electrodes.

Crystalline plates also are mechanical resonators that have natural frequencies of vibration ranging from a few thousand hertz to tens of megahertz. The vibration frequency depends on the kind of crystal, the way the plate is cut from the natural crystal, and on the dimensions of the plate (like the tuning fork and the bell). The thing that makes the crystal resonator valuable is that it has extremely high Q, ranging from 5 to 10 times the Qs obtainable with good LC resonant circuits.


Since the crystal has a definite resonant frequency controlled by the crystal lattice, it can be used to “regulate” an oscillator to a high degree of accuracy.

The crystals we use most often resonate in the 1- to 30-MHz region and are of the AT cut, thickness shear type, although these last two characteristics are rarely mentioned. A 15-MHz-fundamental crystal of this type is about 0.15 mm thick. Because of the widespread use of reprocessed warsurplus, pressure-mounted FT-243 crystals, you may think of crystals as small rectangles on the order of a half-inch in size. The crystals we commonly use today are discs, etched and/or doped to their final dimensions, with metal electrodes deposited directly on the quartz. A crystal’s diameter does not directly affect its frequency; diameters of 8 to 15 mm are typical.

AT cut is one of a number of possible standard designations
for the orientation at which a crystal disc is sawn from the original quartz crystal. The crystal lattice atomic structure is asymmetric, and the orientation of this with respect to the faces of the disc influences the crystal’s performance. Thickness shear is one of a number of possible orientations of the crystal’s mechanical vibration with respect to the disc. In this case, the crystal vibrates perpendicularly to its thickness. Place a moist bathroom sponge between the palms of your hands, move one hand up and down, and you’ll see thickness shear in action.


From QST January 2001
 

While tuning around 1600 kHz I heard a very weak AM signal ...

Q Larry, K0LWV, asks, “While tuning around 1600 kHz I
heard a very weak AM signal that appeared to be repeating
an announcement about a house for sale nearby. Is this
kind of thing legal?”


A So-called “Talking House” transmitters generate the signals you are hearing, and they are becoming popular among realtors.

When a house comes on the market, the seller’s real estate agent may install one of these transmitters to continuously broadcast a sales pitch about the home. The sign on the front lawn invites drivers to tune to a particular frequency to hear the broadcast. Talking House transmitters are FCC Part 15 devices that do not require licenses to own and operate. (They are in the same class as AM and FM “wireless microphones.”) The Talking House units typically operate above 1600 kHz and have an output of 100 mW or less. Their range is limited to about 1500 feet. Believe it or not, some hard-core broadcast-band DXers attempt to receive these signals at much greater distances, although their success varies!


From QST January 2001
 

I have a dipole cut for 20-meters fed with 45 feet of coax and I’m trying to use it on 40-meters with an antenna tuner...

Q I have a dipole cut for 20-meters fed with 45 feet of coax
and I’m trying to use it on 40-meters with an antenna
tuner. However, I need the full amount of tuner capacitance to get the SWR to 1:1. Does that mean that I am consuming a lot of power in the tuner?


A How much 40-meter power gets to your 20-meter dipole
depends upon both the loss in the tuner and the loss in the
feed line. According to the EZNEC antenna-modeling software, if your dipole is 35 feet above average ground, the feedpoint impedance at 7 MHz is about 13.6 – j1000Ω.


While that doesn’t tell you what you get at the tuner end of
the line, N6BV’s TL and TLW programs (from the ARRL Antenna Book disk) will. If you are using RG-213 coax, the shack-end impedance will be 546 + j806 Ω (SWR 35:1). The same programs also give you the total feed line loss for a given SWR. In this case, the line loss is 16 dB. Clearly, the loss in the tuner is not all you need to worry about!


If you substitute 450-Ω ladderline instead, the shack-end impedance is 14 + j667 Ω (SWR 115:1), but the feed line loss drops to 3 dB. Why is it lower than the coax if the SWR is higher? The answer is that the additional loss due to the SWR is proportional to the line’s characteristic loss, and ladderline has much less loss than coax. A 2.7-dB loss is half of your power, though. If you operate on 40 meters a lot you might want to consider a longer antenna (an 80-meter dipole on 40 meters gives a total feed line loss of less than 0.5 dB with 45 feet of ladderline). 

Concerning the loss in the tuner, every tuner design will have a certain amount of loss. Some tuners are more lossy than others. 

From QST January 2001
 

When operating PSK31 I notice that some hams type their text in all uppercase letters ...

Q Larry, WA5MHE, asks, “When operating PSK31 I notice that some hams type their text in all uppercase letters, apparently unaware that the PSK31 code supports upper and lower case. Why do they do this?”

A I suspect that some of these operators may be RTTY veterans. The RTTY code used by most amateurs in the United States is known as ITA No. 2. With the limitations of a
5-bit code, ITA No. 2 can only support a relatively short list of characters. Therefore, RTTY text is in all-uppercase letters, rather than the mix of upper and lower case that we are accustomed to seeing.

There are three problems with sending text in all uppercase:
(1) It is more difficult to read, (2) in the age of the Internet the custom is to interpret all-upper-case words as SHOUTING and (3) uppercase characters in PSK31 take longer to send. Internet savvy hams (the majority of us, these days) are becoming more sensitive to the use of upper and lowercase in digital communication. I think this issue will resolve itself over time.

From QST January 2001