{"id":81,"date":"2014-11-06T06:14:19","date_gmt":"2014-11-06T06:14:19","guid":{"rendered":"http:\/\/vk8bn.me\/info\/?p=81"},"modified":"2015-01-17T00:37:17","modified_gmt":"2015-01-17T00:37:17","slug":"using-cheap-catv-75%cf%89-cable-to-feed-amateur-antenna-systems","status":"publish","type":"post","link":"https:\/\/vk8bn.me\/info\/using-cheap-catv-75%cf%89-cable-to-feed-amateur-antenna-systems\/","title":{"rendered":"Using Cheap CATV 75\u2126 cable to feed Amateur Antenna Systems"},"content":{"rendered":"

RG6 type CATV cable is very cheap compared with price of RG-8 or RG-213 type cables normally used by the majority of amateurs to feed their antennas. RG 6 can be found at electrical wholesalers for as little as $40 for a 100 meter roll.
\nThe Beldon datasheets lists RG213 as having a loss of 1.9dB per hundred feet at 100MHz and RG-6 as having a loss of 1.95 dB per hundred feet at 100MHz.
\nThe problem is the 75\u2126 characteristic impedance, or is it really a problem?
\nMost amateur antenna systems can be adjusted to present a reasonable match to 75 ohms; slightly altering the length of a half wave dipole can result in a near perfect match. If you happen to be using a transceiver with tube output the Pi coupler can be adjusted to perfectly match the 75\u2126 feed line.
\nNot all SWR meters are capable of switching to measure SWR at impedances of anything other than 50\u2126, some however do; I have a Kyoritsu meter that does, and the good old Osker Block that many amateurs have is also capable of being switched to 75\u2126. So in conjunction with a transceiver with tube output and a suitable SWR meter there are no impediments to using cheap RG6 cable and obtaining the same results as using much more expensive 50\u2126 cable.
\nNow I should also point out that the mismatch presented to a 50\u2126 system with a 75Ohm load is only 1.5:1 which will result in a reflected power of 4%. Pr \/ Pi = [(SWR – 1) \/ (SWR + 1)]2 = 0.5 \/ 2.5 =0.04. So it will probably not be an issue using the average solid state transceiver either without doing anything.
\nHowever if you do not have an impedance switchable SWR meter there is still another solution. A broadband impedance transformer wound on a decent sized torroid.
\nA 50\u2126 to 75\u2126 transformation can be achieved by winding 11 turns of wire on a torroid and tapping at two turns this will give a ratio of 9 to 11 or a turn\u2019s ratio of 1.222 to 1.
\nThe impedance ratio of a transformer is the square of the turn\u2019s ratio, so this arrangement will result in an impedance ratio of 1.494:1 close enough to the required 1.5:1.<\/p>\n

 <\/p>\n

\"Image2\"<\/a><\/p>\n

Schematic<\/p>\n

The two turns on the top winding 9 turns on the lower winding.
\nThe constructed transformer when measured with an Autec RX Vector Analyst was found to be slightly inductive and I was able to improve the performance and obtain an SWR of less than 1.2:1 from 1.8 MHz to 21MHz by placing a 100Pf 1Kv capacitor across the 50 Ohm terminals. My device started to degrade above 26 MHz, possibly due to the torroid I selected.
\nI note that low power commercial units are advertised as covering from 1 to 1500 MHz, obviously using a better ferrite than the one from my junk box.<\/p>\n","protected":false},"excerpt":{"rendered":"

RG6 type CATV cable is very cheap compared with price of RG-8 or RG-213 type cables normally used by the majority of amateurs to feed their antennas. RG 6 can be found at electrical wholesalers for as little as $40 for a 100 meter roll. The Beldon datasheets lists RG213 as having a loss of […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[12],"tags":[],"class_list":["post-81","post","type-post","status-publish","format-standard","hentry","category-antennas"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p8ympN-1j","jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/posts\/81","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/comments?post=81"}],"version-history":[{"count":6,"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/posts\/81\/revisions"}],"predecessor-version":[{"id":171,"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/posts\/81\/revisions\/171"}],"wp:attachment":[{"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/media?parent=81"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/categories?post=81"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vk8bn.me\/info\/wp-json\/wp\/v2\/tags?post=81"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}