{"id":1690,"date":"2010-10-07T10:46:58","date_gmt":"2010-10-07T14:46:58","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=3928"},"modified":"2023-07-26T17:04:06","modified_gmt":"2023-07-26T21:04:06","slug":"the-folded-unipole-antenna","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2010\/10\/the-folded-unipole-antenna\/","title":{"rendered":"The folded Unipole antenna"},"content":{"rendered":"\n

In the 1990s, the folded unipole antenna was touted by many to be the savior of AM radio. \u00a0There were many claims that a folded unipole antenna<\/a> did not need a complicated ground system, a simple ground rod at the base of the tower would work fine. \u00a0That turned out to be not exactly the case. \u00a0 Kintronic did a study<\/a> (.pdf)\u00a0that basically dispelled that notion, along with several others. \u00a0 The folded unipole antenna performed within a few percentage points of a series-fed tower under the same testing conditions.<\/p>\n\n\n\n

\"three<\/a>
three wire folded unipole on a guyed tower<\/figcaption><\/figure>\n\n\n\n

Folded unipoles do have the advantage of a grounded tower. \u00a0Grounded towers have a distinct advantage in lightning-prone areas, such as central Florida. \u00a0I can attest through my own experience, a series-fed tower is much more likely to induce lightning damage to a transmitter or ATU. \u00a0Folded unipole tower systems can also be used to co-locate other antennas, such as STL, cellular, PCS, etc. \u00a0Making some extra rental money on an AM tower is not a bad way to go.<\/p>\n\n\n\n

I began fooling around with MANNA-GAL<\/a>, which is a NEC-2-based program. \u00a0It is a free ham radio program, so it is a little clunky to use and it took a while to figure out, but once I did, it is fun. \u00a0I modeled a unipole antenna for medium wave use and the results are pretty interesting. \u00a0First of all, I drew out X-Y part of the system on graph paper because the program requires all wires (elements) to be entered in a coordinate-based format. \u00a0The Z axis is the tower, since there is only one of those, that was easy. \u00a0I played around with series vs. unipole systems and the results were fairly close to what they are supposed to be. \u00a0One of the nice things about MANNA-GAL is it allows the user to change the ground conditions. \u00a0To add a unipole to the tower, I put 3 wires spaced one to two meters away from the primary Z-axis wire, connected them to the top of the tower, and changed the drive point to the skirt wires.<\/p>\n\n\n\n

The interesting part is when I added an above-ground counterpoise instead of a buried radial ground system. \u00a0I think Ron Nott, of Nott, ltd. did much of this work too. \u00a0What I found was that with between 5 – 10 above-ground radials of 90 degrees or greater, the efficiencies are within about 10 percent of theoretical for a 120-buried radial system. \u00a0Again, the ground conductivity plays a big role in this, poor ground conductivity will reduce efficiencies equally for both systems.<\/p>\n\n\n\n

As the tower height approaches 110 degrees or so, depending on the spacing from the tower of the skirt wires, the bandwidth really starts to open up. \u00a0At 110 degrees the base impedance is about 120 ohms with about 80 ohms inductive reactance. \u00a0 Both the impedance and reactance slope slightly upward with frequency but are linear +\/- 50 KHz of the carrier. \u00a0This slight asymmetrical sideband distribution can be easily canceled out in the ATU with a few degrees of negative phase shift through the T network.<\/p>\n\n\n\n

Again, all of this is theoretical, but I have found that NEC is usually within +\/- 10% of real-world values. \u00a0It is difficult to get a handle on ground conductivity unless measurements are taken. \u00a0Even from season to season, that can change.<\/p>\n\n\n\n

The above-ground counterpoise requires a partial proof, according to FCC 73.186. \u00a0If this were a directional station, this would be required anyway. \u00a0For a non-directional station, it is pretty easy, for six radials, it would probably take about one to two days of driving around with a FIM 41. \u00a0The other consideration is public exposure to RFR from the radials. \u00a0This can easily be measured with a NARDA meter. \u00a0More radials will spread the induced currents out more, for higher-powered stations, 10 above-ground radials might be required.<\/p>\n\n\n\n

There are several radio stations in the country that are successfully using above-ground counterpoises. \u00a0It seems to be a good system and requires much less material and labor to install than the traditional ground system.<\/p>\n\n\n\n

Therefore, if I were designing a new AM station, I’d use a grounded tower between 105 and 110 degrees with a unipole and 6 above-ground radials 90 degrees or greater.<\/p>\n","protected":false},"excerpt":{"rendered":"

In the 1990s, the folded unipole antenna was touted by many to be the savior of AM radio. \u00a0There were many claims that a folded unipole antenna did not need a complicated ground system, a simple ground rod at the base of the tower would work fine. \u00a0That turned out to be not exactly the … Continue reading The folded Unipole antenna<\/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":[33,18],"class_list":["post-1690","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-am-towers","tag-am-transmitter-site"],"_links":{"self":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/1690","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=1690"}],"version-history":[{"count":8,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/1690\/revisions"}],"predecessor-version":[{"id":12297,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/1690\/revisions\/12297"}],"wp:attachment":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media?parent=1690"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/categories?post=1690"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/tags?post=1690"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}