antenna help needed - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: November 04, 2023, 10:57:23 am
content: i want some ideas of how to make an 80m antenna with what i have,i have limited space,parts to use are 1x 10m fishing pole,1x 7m fishing pole atu- 450 ohm +300 ohm windowline 10m of each,100m of 1mm magnet wire,i want to use the poles back to back horizontaly ie dipole,center fed,i want mostly 80m+up if pos,how can i make this?,has anyone plans i can use?.73

author: iMo
author_rank: Super Contributor
timestamp: November 04, 2023, 11:29:54 am
content: What does it mean "limited space"?Magnet copper wire is not good for a fixed antenna, you have to use something more suitable (an enforced stranded copper wire, with kevlar or steel inside).The shortest wire antenna for 80+ people use is the EFHW (end fed half wave dipole basically) antenna for 80-10m (aprox 21m in length, shorted by an aprox 100uH coil). Works fine, coaxial fed, you would need an unun 1:49 or similar ratio.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 04, 2023, 02:09:04 pm
content: read my post againe the wire will be fixed to the pole so its two vertlicle poles thick end to thick end in a horizontal position center fed.

author: fourfathom
author_rank: Super Contributor
timestamp: November 04, 2023, 03:19:29 pm
content: It sounds like you are contemplating a "continuously loaded" dipole. This is where the loading coil (used in electrically short radiators) is wound along the entire length of the radiator, rather than as a lumped base-loading coil. You usually see these loading coils in short verticals, but there's no reason they can't be used in dipoles. I once made a "Slinky" dipole antenna (https://www.nonstopsystems.com/radio/frank_radio_antenna.htm), hung it on the wall of my apartment bedroom, and used it on 40 meters. It worked, but I suspect it was not very efficient. I have no idea what the antenna impedance was, this was back in the days of vacuum-tube transmitters so I just adjusted the transmitter output controls (dipping and peaking the plate current).The diameter and resistance of the coil is important, and I suspect that using the small-diameter fishing poles and magnet wire will be quite lossy (I have no math to prove this.) But even a lossy antenna can work well enough to make contacts. Some web-searching is probably in order.

author: A.Z.
author_rank: Super Contributor
timestamp: November 04, 2023, 03:50:25 pm
content: Quote from: p.larner on November 04, 2023, 02:09:04 pmstart from the "inverted U dipole" herehttps://www.qsl.net/dk7zb/portabel/inverted_u.htmhttps://www.simplehamradioantennas.com/2013/08/the-bent-dipole-or-inverted-u-antenna.htmlbut instead of a dipole, build a doublethttps://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htmhttp://on5au.be/content/a10/wire/abd.htmlfeed it with twin line to the bottom of the support pole, connect the remote ATU there and then run coax to the shack

author: A.Z.
author_rank: Super Contributor
timestamp: November 04, 2023, 04:15:26 pm
content: Here's an alternate idea for you, let's say we remove the thinner top elements from both rods and remain with 9m and 6m, now, we start by finding a good lenght herehttps://udel.edu/~mm/ham/randomWire/and one which covers the desired bands, then we put the wire in inverted L, assuming the two rods are supported by a pair of 1m metal poles, we put the remote ATU near the top of the metal support pole, from there, our chosen lenght of wire will go up vertically for 9m and the remainder of it will slope down to 6m, that is the second rod placed at a distance, add some radials laying on ground, and then a choke and the coax all the way from the remote ATU to the shackSuch an antenna would cover "all bands" and probably will have better takeoff angle than the horizontal doublet, in particular at lower frequencies

author: p.larner
author_rank: Frequent Contributor
timestamp: November 05, 2023, 10:01:02 am
content: firstly,i have no room for any radials,hence the dipole idea,i have just enough room to get a 20m span over part of my roof ,i was thinking of making each pole loaded for 80 with a coil say 1/3 of the way up its length and do the same to both and have the bottom thick ends joined back to back,basicaly did the same thing with hamsticks but it was a barstuard to tune and was a big dummy load,was thinking of doing the same thing with fishing poles ,reason being it would be more wire in the air so would work better?.

author: A.Z.
author_rank: Super Contributor
timestamp: November 05, 2023, 02:07:48 pm
content: the two rods are of different lenghts, so I suspect the resulting structure will have some mechanical issues, as for loading coils, my rule of thumb is to place them near the far ends of the dipole and then to avoid shortening the dipole more than 40% (the antenna lenght should be about 60% or more of a full size one)

author: p.larner
author_rank: Frequent Contributor
timestamp: November 05, 2023, 04:12:57 pm
content: as i said a 20m span is as far as i can go.I am not to worried about the bw as i just need a spot frequency for a local net on the top end of 80.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 07, 2023, 05:43:52 am
content: lets try and explain this better,viewed from the antenna will look like a flat line,ie 2 ten meter poles joined end to end at there bases,center fed where they join,but need loadng coils for 80m,need to know size and position of coils etc.the poles are about 50mm dia at the bases.,basicaly two verticle antennas joined at the base centerfed on the join in a horizontal orientation.hope that clears things up.

author: A.Z.
author_rank: Super Contributor
timestamp: November 07, 2023, 06:20:21 am
content: https://www.nonstopsystems.com/radio/frank_radio_antenna_80m-dipole.htmhttps://www.k7mem.com/Ant_Short_Dipole.html

author: p.larner
author_rank: Frequent Contributor
timestamp: November 07, 2023, 03:14:17 pm
content: well i have wound a coil of 43uh 5m from the top of one 10m pole,i am going to try just 1x 10m pole as a loaded inverted l fed against ground,i can get approx 20m of wire,hope with the coil halfway up the pole it will tune on 80 using my atu?.

author: A.Z.
author_rank: Super Contributor
timestamp: November 09, 2023, 11:13:31 am
content: Quote from: p.larner on November 07, 2023, 03:14:17 pmForgive me, but you started insisting about an horizontal dipole, saying that you had no room for a vertical, and now you suddenly changed your mind and write you built an inverted L... please choose an antenna and go on with it; as for the inverted L, it can be fed against ground, although losses will be higher than using radials, and if going for a resonant antenna for 80m, you'd better place the coil along the "horizontal" arm of the L (at about 2/3 length from the feedpoint) since the most radiation will come from (about) the first 60% of the radiator wire from the feedpoint, so moving the loading coil far away from it will improve radiation efficiencyAnyhow, here's another idea for you, the vertical radiator may be a metal pipe, while the hats may be supported by four 2m fishing rods placed "back to back", the pic shows the details as simulated in NEC, the matching isn't perfect, but is ok and then an ATU in the shack will adjust it; as for the vertical pipe, it may be built in two or three sections so that it will be easier to adjust the length to tune the antenna

author: srfsteleinfra
author_rank: Newbie
timestamp: November 23, 2023, 07:33:58 am
content: RF Antenna help needed: Seeking guidance on selecting the bestRF antennafor optimal signal reception. Factors like frequency range and directional capabilities are crucial. Any suggestions or advice on reputable brands providing reliable RF antennas?