Improv­ing an anten­na sys­tem is one of the most pro­duc­tive moves open to the VHF enthu­si­ast. It can increase trans­mit­ting range, improve recep­tion, reduce inter­fer­ence prob­lems and bring oth­er prac­ti­cal ben­e­fits. The work itself is by no means the least attrac­tive part of the job. Even with high-gain anten­nas, exper­i­men­ta­tion is great­ly sim­pli­fied at VHF and UHF because an array is a work­able size, and much can be learned about the nature and adjust­ment of anten­nas. No large invest­ment in test equip­ment is nec­es­sary.

 

Whether we buy or build our anten­nas, we soon find that there is no one best design for all pur­pos­es. Select­ing the anten­na best suit­ed to our needs involves much more than scan­ning gain fig­ures and prices in a manufacturer’s cat­a­log. The first step should be to estab­lish pri­or­i­ties for the anten­na sys­tem as a whole. Once the objec­tives have been sort­ed out in a gen­er­al way, we face deci­sions on spe­cif­ic design fea­tures, such as polar­iza­tion, length and type of trans­mis­sion line, match­ing meth­ods, and mechan­i­cal design.

 

As has been dis­cussed pre­vi­ous­ly, shap­ing the pat­tern of an anten­na to con­cen­trate radi­at­ed ener­gy, or received sig­nal pick­up, in some direc­tions at the expense of oth­ers is the only pos­si­ble way to devel­op gain. Radi­a­tion pat­terns can be con­trolled in var­i­ous ways. One is to use two or more dri­ven ele­ments, fed in phase. Such arrays pro­vide gain with­out marked­ly sharp­en­ing the fre­quen­cy response, com­pared to that of a sin­gle ele­ment. More gain per ele­ment, but with some sac­ri­fice in fre­quen­cy cov­er­age, is obtained by plac­ing par­a­sitic ele­ments into a Yagi array.

 

Anten­na radi­a­tion can be made omni­di­rec­tion­al, bidi­rec­tion­al, prac­ti­cal­ly uni­di­rec­tion­al, or any­thing between these con­di­tions. A VHF net oper­a­tor may find an omni­di­rec­tion­al sys­tem almost a neces­si­ty but it may be a poor choice oth­er­wise. Noise pick­up and oth­er inter­fer­ence prob­lems tend to be greater with omni­di­rec­tion­al anten­nas. Max­i­mum gain and low radi­a­tion angle are usu­al­ly prime inter­ests of the weak-sig­nal DX aspi­rant. A clean pat­tern, with low­est pos­si­ble pick­up and radi­a­tion off the sides and back, may be impor­tant in high-activ­i­ty areas, where the noise lev­el is high, or for chal­leng­ing modes like EME (Earth-Moon-Earth).

 

In gen­er­al, the high­er a VHF anten­na is installed, the bet­ter will be the results. If rais­ing the anten­na clears its view over near­by obstruc­tions, it may make dra­mat­ic improve­ments in cov­er­age. With­in rea­son, greater height is almost always worth its cost, but height gain must be bal­anced against increased trans­mis­sion line loss. Line loss­es can be con­sid­er­able at VHF and above, and they increase with fre­quen­cy. The best avail­able line may be none too good, if the run is long in terms of wave­length. Con­sid­er line loss­es in any anten­na plan­ning.

 

A giv­en anten­na design for 432 MHz, say a 5‑element Yagi on a 1‑λ boom, will have the same gain as one for 144 MHz, but being only one-third the size it will inter­cept only one-ninth as much ener­gy in receiv­ing. Thus, to be equal in com­mu­ni­ca­tion effec­tive­ness, the 432-MHz array should be at least equal in phys­i­cal size to the 144-MHz one, requir­ing rough­ly three times the num­ber of ele­ments. With all the extra dif­fi­cul­ties involved in going high­er in fre­quen­cy, it is well to be on the big side in build­ing an anten­na for the UHF bands.

 

Whether to posi­tion the anten­na ele­ments ver­ti­cal­ly or hor­i­zon­tal­ly has been a ques­tion since ear­ly VHF oper­a­tion. Orig­i­nal­ly, VHF com­mu­ni­ca­tion was most­ly ver­ti­cal­ly polar­ized, but hor­i­zon­tal gained favor when direc­tion­al arrays became wide­ly used. Tests of sig­nal strength and range with dif­fer­ent polar­iza­tions show lit­tle evi­dence on which to set up a uni­form polar­iza­tion pol­i­cy. On long paths there is no con­sis­tent advan­tage, either way. Short­er paths tend to yield high­er sig­nal lev­els with hor­i­zon­tal in some kinds of ter­rain. Man-made noise, espe­cial­ly igni­tion inter­fer­ence, tends to be low­er with hor­i­zon­tal polar­iza­tion. Ver­ti­cal­ly polar­ized anten­nas, how­ev­er, are marked­ly sim­pler to use in omni­di­rec­tion­al sys­tems and in mobile work, result­ing in a stan­dard­iza­tion on ver­ti­cal polar­iza­tion for mobile and repeater oper­a­tion on FM and for dig­i­tal com­mu­ni­ca­tions. Hor­i­zon­tal polar­iza­tion is the stan­dard for weak sig­nal VHF and UHF oper­a­tion. (Cir­cu­lar polar­iza­tion is pre­ferred for satel­lite work as described below.) A loss in sig­nal strength of 20 dB or more can be expect­ed with cross-polar­iza­tion so it is impor­tant to use anten­nas with the same polar­iza­tion as the sta­tions with which you expect to com­mu­ni­cate.


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