1/8 wave - Page 1
Forum section: electronics / rf-microwave

author: daslolo
author_rank: Regular Contributor
timestamp: October 16, 2019, 10:11:00 pm
content: I'm considering moving to 1.2Ghz video transmission because penetration through woods is better but the antennas are too large.So I'm wondering, what's the loss of signal when switching from 1/4 wave antennas (1.2Ghz size) down to 1/8 wave (2.4Ghz size)?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: October 17, 2019, 01:52:37 am
content: 12.5Cm is to large? To see the effects of what different length radiators will do you could always fire up some antenna stimulator and simulate the effects of different typologies on radiation pattern and effective radiated power. Whether something is good enough, really depends on the kinds of girls you go out with and if she suits the application.

author: TheSteve
author_rank: Unknown
timestamp: October 17, 2019, 02:08:00 am
content: Just using an 1/8th wave will suck. Hard to believe a 12.5cm wire is too large.

author: bob91343
author_rank: Super Contributor
timestamp: October 17, 2019, 02:17:15 am
content: Antenna impedance becomes capacitive. The resistive component is reduced. Thus it's harder to drive the same amount of power. One can add inductance to cancel the capacitive reactance but is still left with the reduced resistive component. If you can drive it, it will work.Antenna pattern will change. Much depends on your configuration. A counterpoise will be good. But the angle of radiation will change. Since the current will be higher for the same power, resistive losses will increase.

author: radiolistener
author_rank: Super Contributor
timestamp: October 17, 2019, 04:50:11 pm
content: Quote from: daslolo on October 16, 2019, 10:11:00 pmIt will have impedance about 6.71 - j181.78 ohms. I'm afraid it will very hard to match such antenna with transmitter. You will spend much more on impedance match circuit than on buy proper antenna. And efficiency of such system with impedance match circuit at 1.2 GHz will be awful due to high loss.

author: daslolo
author_rank: Regular Contributor
timestamp: October 18, 2019, 09:31:45 pm
content: Got it thanks.How can antenna be mignaturized?I notice that antennas on a cell phones are tiny, far smaller than 1/4 wave of 1000-2000 Mhz, is there any way to be half way in term of size and retain decent range and keep the high penetration of 1.2Ghz?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 19, 2019, 01:40:34 am
content: If it is a vertical antenna like those mounted on a connector at the rear of various devices, it is not uncommon for them not to be straight 1/4 wave antennas, but much shorter, but with 1/4 wavelength of wire wound upon them.This is not dissimilar to base loading wih inductance, except thst the helically wound conductor contributes to the desired signal radiation.Common examples of such antennas are the WIFI antennas on the back of ADSL boxes, & the "rubber ducky" antennas on handheld two way radios.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 19, 2019, 01:51:50 am
content: Quote from: TheSteve on October 17, 2019, 02:08:00 amIt's even smaller than that ------- 12.5 cm is ahalfwavelength at 1200MHz.A 1/4 wavelength at that frequency is 6.25 cm in free space, & allowing for "end effect", the antenna ends up being 5.94 cm.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 19, 2019, 01:57:12 am
content: Quote from: daslolo on October 16, 2019, 10:11:00 pmWhen you say "penetration through woods", do you mean the place where the"Big Bad Wolf" lives, or wood used structurally in your house/workplace?

author: daslolo
author_rank: Regular Contributor
timestamp: October 19, 2019, 02:25:20 am
content: "big bad wolf"if I roll a dipole in a spiral won't that require the emitting antenna to also be a spiral?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 19, 2019, 05:29:09 am
content: Quote from: daslolo on October 19, 2019, 02:25:20 amThere are two distinct types of antenna commonly called "helical", the "Normal mode" (here, "normal" is used in its geometric sense) and the "Axial mode"These are described in the Wiki :-https://en.m.wikipedia.org/wiki/Helical_antennaI have also linked a site which seems to be dealing with very (physically) small "Normal mode"antennas.https://www.hindawi.com/journals/ijap/2018/7967468/I'm a bit dubious about using a smaller antenna if you have to send such a signal through a wood, as most people go for gain antennas in such circumstances.A few years ago, I tried to receive a ham radio signal on 1296 MHz, & despite a fairly good VHF/UHF site, & the higher power allowed for hams, I heard not a "sniff".I was using a scanner with, initially its"rubber ducky" antenna, & then with a homemade 1/4 wave groundplane, to no avail.I knew when they were transmitting, as it was basically a test of their equipment, so they also had a "back channel" on 2m.Other people, with gain antennas received the 1296MHz signal.

author: bob91343
author_rank: Super Contributor
timestamp: October 19, 2019, 06:40:58 am
content: As an old time ham operator, I will state that a small antenna is not a good antenna. If you make the antenna smaller than the classical dimensions, the impedance and pattern and losses all change in ways that frustrate your intentions.Shortening an antenna, as I mentioned above, benefits from a series inductance to cancel the capacitive component. Stepping down the voltage from the source will drive the new lower impedance better than otherwise.Realize that an antenna is part of a circuit. So if current is to flow in it, there has to be a return path. This is where a counterpoise can help. Or use a balanced configuration, which doubles the size.Loop antennas can be small in terms of their overall dimension but will take larger area.This subject is not a simple one. Many books have been written and many clever designs invented. You can start with a classic book called 'Antennas' written by a master of the subject, John Kraus. This book was written many years ago but its principles, like Ohms's law, still prevail.At one time I would have recommended the Radio Amateur's Handbook or even the ARRL Antenna Book but no more; these have deteriorated to the point of misinformation. There are some engineering periodicals that occasionally treat antennas but even these have dropped in credibility. Two that come to mind are 'Microwaves and RF" and 'High Frequency Engineering'. I may have screwed up some of the titles but I hope you get the idea.The bottom line is that there is no simple answer to the question. It depends on required antenna gain, size, impedance, pattern, bandwidth, and a bunch of other factors. Not to mention coupling methods. Or antenna arrays.

author: daslolo
author_rank: Regular Contributor
timestamp: October 21, 2019, 02:14:31 am
content: Quote from: bob91343 on October 19, 2019, 06:40:58 amThat was my first hunch. My understanding, when I gave up that idea, was that it requires expensive RF chips to do the MIMO magic. So in a way convert hardware dimensions into IC cost.Maybe I'm wrong, can you tell me more?I started to watch Kraus presentations, when I feel I understand enough I may upgrade to book then perhaps I'll understand what you mean by Counterpoise or adding serial inductance (that coil at the base of a whip antenna?)Is there a formula i can apply that connects Hz, length of di/monopole, bandwidth and penetration through wood, leaves?Quote from: vk6zgo on October 19, 2019, 05:29:09 amis the idea to get a coild of the same self resonance Hz as the target Hz? like this youtube.com/watch?v=tjbK4LsOQRkYeah I hear ya, I use patch antennas on my 5.8ghz video system, what i'm trying to do here is determine the right balance between hz size and penetration. for example a 5.8ghz system through the woods will choke after a few meters, not only multipathing but signal will fade. the lower the Hz the more the penetration but also the larger the antenna, until it doesn't fit a mini quad. etc... you get the idea.

author: bob91343
author_rank: Super Contributor
timestamp: October 21, 2019, 05:03:12 am
content: The coil at the base of a vertical antenna, or sometimes halfway up, is there to resonate the capacitance of the antenna (a result of being too short). Capacity hats are also used.These are all to make up for antenna deficiency. In your case, you are starting out with the idea of making a deficient antenna and hoping you will find a way to get it to work. Sure you can make it work but you won't have a good setup. The farther you deviate from the 'proper' design (elements that resonate without help), the poorer the performance.I remember being asked what is a good small antenna that doesn't have to be very high. I responded that a good antenna must be large and high. Anything else is a compromise, and the farther you go, the worse it works.

author: radiolistener
author_rank: Super Contributor
timestamp: October 23, 2019, 11:53:29 pm
content: Quote from: bob91343 on October 21, 2019, 05:03:12 amantenna can be small, but even a small antenna still needs to be placed far away (about half wavelength) from anything.