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CHAPTER
Hidden and limited-space antennas
ONE OF THE MOST SIGNIFICANT IMPEDIMENTS TO AMATEUR RADIO OPERATORS, CB
operators, and shortwave listeners is the space available for their antennas In many thousands of other cases, the limitation is less one of space, but of regulators More and more subdivisions are built with covenants on the deed that prohibit the buyer from installing outdoor antennas Once limited to townhouse developments, where that breed of contemptible vermin (called the Homeowners Committee) routinely intruded on the affairs of people who mistakenly think they own their townhouse (ownership implies right of use, which is limited by the covenants) These onerous covenants are now routinely placed on single-family dwellings as well In fact, it is the single most serious threat to amateur communications people in the country today Other homeowners are no longer angered by the restriction on antennas because their television reception is now carried to them via cable systems in most parts of the country Even where cable is not available, most users can install a moderately sized television antenna in their attic, or use rabbit ears In other cases, the townhouse community will install a single master TV antenna and then distribute signals to each unit The result is that the amateur radio operator, CB operator, and shortwave listener are left to fend for themselves without assistance from neighbors In this chapter we will examine some of the alternatives available to those readers who have either a limited space situation (such as a small city lot), or are unable to move out of a subdivision where there are stupid rules against outdoor antennas The suggestions contained in this chapter are not universal, and indeed the author recommends that you adapt, as well as adopt, these recommendations, and come up with some of your own Creativity within the constraints of the laws of physics governing radio antennas is encouraged
Copyright 2001 - the McGraw-Hill Companies
232 Hidden and limited-space antennas
Hidden antennas
A hidden antenna is one that is either completely shielded from view or disguised as something else Alternatively, you could also include in this category antennas that are in semiopen view, but which are not too obvious (except to the trained and diligent eye) Some people have opted for hidden longwires made of very fine wire (no 26 enameled wire is popular) The user will install the wire in the open, high off of the ground (as in an apartment installation), and operate without anyone knowing the difference One chap used no 22 wire suspended between two 16-story apartment house buildings that were 100 yards apart He had one of the best working longwires in town until a windy day when the whole thing came down No one was injured; but had someone been hurt, there might have been a lawsuit Hidden antennas must be designed with an eye toward causing others no harm It is neither ethical nor smart to place others at risk in enjoying our hobbies The dipole is a popular antenna with both shortwave listeners and amateur radio operators Indeed, for the CB operator who wants to get on the air from an apartment or townhouse (or restricted single-family home), the dipole can represent a respectable alternative As you learned in Chap 6, the dipole is a horizontal wire (or pipe) antenna that is a half-wavelength long and fed in the center (ideally) with 75- coaxial cable In the townhouse dipole, it is possible to build the antenna entirely inside the attic of the building The length of the dipole is given approximately by Lft 468 FMHz [101]
If you do some quick calculations you will find that antennas for the 10-, 13-, 15-, and (possibly) even 18- and 20-m bands, will fit entirely inside the typical townhouse attic This statement is also true of the 11-m citizen s band antenna: It will fit inside the standard townhouse attic But what about the lower frequency bands Figure 10-1 shows a possible solution to the use of the lower frequencies in the townhouse situation The two quarter-wavelength arms of the dipole are ideally installed in line with each other, as was shown in the chapter on horizontal antennas But in a sticky situation we can also install the dipole with the arms bent to accommodate the space available In this example, only one of many possible methods for accomplishing this job is shown Here each quarter-wavelength section is composed of two legs, AB and CD, respectively Ideally, segments B and C are the longest dimensions Also, if possible, make segments A and D equal lengths Another method is to reverse the direction of one end leg, say for example D, and run it to the other corner of the building over the peak of the roof The author is almost hesitant to offer only one drawing, despite space constraints, in the fear that readers will take the offered pattern as the only authorized version, or solution In reality, you might not be able (for a variety of reasons) to use the exact pattern shown (so ad lib a little bit) How about performance Will the constrained dipole of Fig 10-1 work as well as a regular dipole installed a wavelength or two off the ground and away from objects
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