{"id":649,"date":"2010-03-15T17:54:16","date_gmt":"2010-03-15T21:54:16","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=3928"},"modified":"2023-07-09T15:16:52","modified_gmt":"2023-07-09T19:16:52","slug":"broadbanded-am-antennas","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2010\/03\/broadbanded-am-antennas\/","title":{"rendered":"Broadbanded AM antennas"},"content":{"rendered":"\n

Partly for my own edification, partly just because, here is some information about AM antenna systems and their bandwidth. An AM tower is a radiator that, simply by the physical constraints of the tower structure itself, is pretty narrow-banded, even under the best conditions. Add to that, antenna tuning units, transmission line phasing, antenna phasing units, diplexing units, and things can get very squished outside of the immediate carrier frequency. This seems to be a particular problem with directional antennas, which most AM stations employ.<\/p>\n\n\n\n

\"WGY<\/a>
WGY 810 kHz, Schenectady, NY transmitting tower with open transmission line<\/figcaption><\/figure>\n\n\n\n

As an engineer, you can get some idea of how narrow an antenna system’s bandwidth is by looking at the base impedance measurement. \u00a0Every AM station is required to keep the latest impedance measurement on file. \u00a0When looking at these measurements, there will be one curve that indicates base resistance (R) and another curve that indicates reactance ( X, although often noted as + or -j). \u00a0If the resistance and or reactance curve is slopped steeply at the carrier frequency and out to 20-30 kHz, it is a narrow tower. \u00a0Add to that the different phase shifts of an ATU and or Phasor and things will be compounded. \u00a0That is why it takes a professional to design and tune up these things, a poor design will never sound right.<\/p>\n\n\n\n

Another way to get some idea of bandwidth requires a field strength meter. \u00a0Modulate the transmitter with a 10 kHz tone at 50% modulation. \u00a0Then, away from the near field, measure the carrier and 10 kHz +\/- the carrier frequency on the log scale. \u00a0The sidebands should be symmetrical and about 1\/4 the carrier level.<\/p>\n\n\n\n

Generally speaking, antenna systems need to be designed for low VSWR across the entire side band range (+\/- 10 kHz from the carrier) as well as symmetrical distribution of radiated energy across the lower and upper sidebands. \u00a0Several factors influence these conditions:<\/p>\n\n\n\n

    \n
  1. Electrical tower height is perhaps the hardest thing to change once a tower is constructed. \u00a0Short towers (less than 80 electrical degrees), or very tall towers, (taller than 200 electrical degrees) present problems. \u00a0If one were constructing an AM station and could choose any tower height, something between 120 to 190 electrical degrees would be ideal. \u00a0Existing towers can be top-loaded to add electrical height for an additional 30 degrees or so. \u00a0Beyond 30 degrees it becomes difficult to physically attain and therefore impractical in most situations. \u00a0Top loading and bottom loading of a tower can reduce bandwidth if done improperly. \u00a0 Bottom loading an AM tower is almost never done due to the very high voltage and current as the electrical length approaches 180\u00b0.<\/li>\n\n\n\n
  2. Antenna matching networks can greatly improve or degrade bandwidth, depending on how they are designed. \u00a0A T-matching network has more parts and is more expensive, however, it allows for optimum control over the R and jX phasing. \u00a0This becomes much more difficult with directional antenna where phase considerations are a part of the station’s antenna field pattern development.<\/li>\n\n\n\n
  3. Phasors present the biggest challenge, particularly in the power divider sections. \u00a0A tank circuit power divider is the worst choice, and a shunt circuit power divider is the best bandwidth choice, however, it is the hardest to conceptualize.<\/li>\n<\/ol>\n\n\n\n

    Obviously, the more complicated the antenna system, the harder it will be to keep the bandwidth open over 20 kHz of spectrum. \u00a0This is especially true on lower-frequency AM signals, where the bandwidth is a much larger percentage of the frequency. \u00a0 Multiple patterns, multiple tower DAs are a nightmare. \u00a0Single-tower non-directional stations are the easiest to modify.<\/p>\n\n\n\n

    As far as the circuit itself, higher Q<\/a> circuits have smaller bandwidths. \u00a0Simply stated, in an alternating current circuit, Q=X\/R. \u00a0The better the reduction of X, which also has a lot to do with the relationship of the current and voltage phasing, the better the Q will be. \u00a0This is why a T network is the best design for an ATU. \u00a0With a 90\u00b0 or 180\u00b0 tower, this is relatively straightforward. \u00a0In towers that are shorter or taller than that, it becomes more difficult as the value of R becomes less friendly.<\/p>\n\n\n\n

    In most cases, some sort of L\/C network can be deployed to decrease the Q of an antenna system at the base of the tower. \u00a0Directional stations also need to have the phasing equipment looked at, because, as noted above, certain designs can create bandwidth bottlenecks. \u00a0All in all, it is usually an expensive proposition for a multi-tower directional station to broadband its antenna system. \u00a0This is another reason why IBOC on AM is destined to fail, many AM towers cannot pass the extended sidebands adequately.<\/p>\n","protected":false},"excerpt":{"rendered":"

    Partly for my own edification, partly just because, here is some information about AM antenna systems and their bandwidth. An AM tower is a radiator that, simply by the physical constraints of the tower structure itself, is pretty narrow-banded, even under the best conditions. Add to that, antenna tuning units, transmission line phasing, antenna phasing … Continue reading Broadbanded AM antennas<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[33,32],"class_list":["post-649","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-am-towers","tag-am-transmitters"],"_links":{"self":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/649","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/comments?post=649"}],"version-history":[{"count":8,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/649\/revisions"}],"predecessor-version":[{"id":12124,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/649\/revisions\/12124"}],"wp:attachment":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media?parent=649"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/categories?post=649"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/tags?post=649"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}