antenna lenth - Page 1
Forum section: electronics / rf-microwave

author: m3vuv
author_rank: Super Contributor
timestamp: June 20, 2020, 11:07:07 am
content: Hi all,ive no room for an 80m antenna as live in a flat,however i can sling a wire over the roof,i can use my pipework as an rf ground,ie one leg of a dipole if you like,my question is what would a good starting lenth be for the driven leg be(ish) as a ball park figure?,i have a 4-1 and a 9-1 un un i can use,my rig is a ts-850 with an atu,i need it for inter g work,ie nvis,just need a starting point that is within the range of my radio atu,73 thanks in advance.,just to say the copper water pipes in the property are all linked so should be ok as one leg/counterpoise i imagine.

author: m3vuv
author_rank: Super Contributor
timestamp: June 22, 2020, 12:36:29 pm
content: no replies,supprising.

author: TheMG
author_rank: Frequent Contributor
timestamp: June 23, 2020, 04:05:48 am
content: I assume the length of the wire you're able to put out onto the roof will be less than a quarter wavelength?I would just put the wire up, as long as you can practically make it, then see how it tunes up. Easiest if you have access to an antenna analyzer to measure the impedance at the desire frequency otherwise pretty much trial and error.Keep in mind that as your wire gets shorter than 1/4, the impedance will drop, and as it exceeds 1/4 wavelength, the impedance goes higher. Your ununs may help the tuner achieve a match by transforming the impedance either up or down as required.None of this is an exact science since the wire antenna will have a counterpoise with relatively unknown characteristics, and it will be in relative proximity to a number of objects (the roof and building) which will affect it's characteristics and resonances.So the best thing I can say is: try it out and experiment.

author: m3vuv
author_rank: Super Contributor
timestamp: June 24, 2020, 08:06:31 am
content: The issue here is i only realy have one shot at this,so need the approx lenth of the drven leg if you like,should i make it 18.89meters as a starting point?also as im using pvc covered wire what % should i shorten it due to the pvc loading?,i have a 4-1 unun and a 9-1 unun i can use its for a spot frequency of 3.775 Mhz.73 m3vuv

author: CaptDon
author_rank: Super Contributor
timestamp: June 24, 2020, 02:10:35 pm
content: One thing to be on the lookout for is what wiring is under the roofand may be close to your antenna wire. If you get anywhere neara resonant 1/4 length of wire there will be hundreds of volts of RFon the tip of the wire. You will also induce strong magnetic fieldsto anything near the feed point. I ran a longwire for 80m to supportthe testing of a local field day event (2019 Conneaut Ohio D-Dayre-enactment) and at 50 watts into the longwire I was getting RFburns from the metal hanger clip on the microphone and I alsokilled a screw-in LED lamp (60 watt equivalent bulb) in the ceilinglamp fixture. It flickered once or twice and went out never to lightagain. I was running 50w AM on 3.885mhz and apparently me andthe house wiring were receiving most of the radiated output!!

author: cdev
author_rank: Super Contributor
timestamp: June 27, 2020, 02:55:42 pm
content: Clip on ferrites could likely help somewhat if a ham is constrained to only having an antenna on a balcony (a loop antenna for example) or rooftop.LED lighting likely also creates a lot of RFI and so I bet that makes a wire on a rooftop especially not a very useful antenna for that reason. Receive noise.A magnetic loop is better (inherently a markedly more quiet antenna than many) but the problem of high RF is going to be a real one for any nearby equipment or wires etc.As much distance as possible and Lots of ferrites is what I would try if that turns out to be happening. Lots and lots of them inside the walls and on coax to sponge up the RF common mode currents on wiring.

author: A.Z.
author_rank: Super Contributor
timestamp: June 29, 2020, 01:07:34 pm
content: Quote from: m3vuv on June 20, 2020, 11:07:07 amHello, would you please give the approximate available space ? See, to suggest you an antenna it would be useful knowing how large/long is the space you've available; as a generic and totally unoptimized solution to your question, if you can't put up a vertical on the roof, I'd suggest considering a random wire antenna; in such a case you may pick the longest wire fitting your space from this sitehttp://www.hamuniverse.com/randomwireantennalengths.html(the green numbers), and consider that to be able to work a given band with some efficiency the wire should be longer than 1/4 wavelenght, so for example the shorter length from the above site which allows to cover the 40m band is 35.5ft; then you'll need a 9:1 UnUn to feed the antenna using coax and you'll also need a good ground and some chokes on the feedline (guanella chokes will fit) otherwise, not only your feedline will radiate, but you'll also have a lot of common mode issues and "RF in the shack" (which is bad, mind me); another antenna which you may consider is the "OCEF" presented herehttps://www.qsl.net/k4fk/presentations/2014-Jul-15-Improvised-antennas-for-emergencies.pdfwith some adjustments it may allow you to get on the air (I'd change the design by adding a twin core 4:1 balun at the feedpoint, but then that's just me)As I wrote at the beginning, having a better idea of the available space would allow to offer some better ideas

author: worsthorse
author_rank: Super Contributor
timestamp: June 30, 2020, 02:14:58 am
content: You might try and end-fed random length wire antenna. There is a lot of info on the web about these kinds of antennas, including tables showing random lengths that will work for multiple bands. This was the first antenna I put up and I used it for a couple of years.For 80m to 10m, IIRC, 87 feet is the magic length, along with a counterpoise of 15 to 30 feet. Again, there is lots of info on the web about this. You will need an impedance matching device (an "antenna tuner) to make this wire work across band.One more thing... such antennas don't have to be straight, though bends do change the radiation pattern. My last version of this antenna started at 11 feet above ground, went to a tree about 30 feet away at 30 feet up and made a hard left to end up at another tree. I've used them in an inverted "L" form, too. Not ideal but a good, easy antenna with which to get started.

author: cdev
author_rank: Super Contributor
timestamp: July 01, 2020, 10:09:47 pm
content: a 43 foot long vertical seems to be extremely popular with a lot of people, here in the US paired with a unun. But if you sling it over your roof your neighbors (or you) are going to be getting that RF - it might even damage costly equipment, like an HDTV, if one is right below it.you dont want to get hit with a bill for that.