When building antenna do you use the speed of light in vacuum or copper (0.66)

[Beamin]

For the longest time I read that 1/4 wave antennas were 0.25m for ~300 MHz and cutting mine accordingly. Should that be times 1/0.66 longer since the speed of light in copper is slower? Most antenna guides mention this but then seem to ignore it or perhaps my feet to metric is wrong. I would imagine setting SWR on transmit would be a nightmare if everything was off by 1/3rd.

[martys]

https://lowpowerlab.com/guide/rf-best-practices/velocity-factor/

Velocity factor of pure copper wire  = .95       not .66 SOL.

[T3sl4co1l]

Velocity in wire is very low, because the permeability is 1 and the permittivity is j1000s give or take.  This gives rise to skin effect, and is why it's so shallow: the index of refraction is huge.

Fortunately for antennas, this means that much more of the EM field is forced out of the wire, into the space around it, where its velocity is more or less the speed of light.

0.66 velocity factor is true for certain types of coax (the cheapest, commonest types), where the above is still true, but the permittivity of the space between conductors is not that of free space, but filled with a dielectric.  In fact, 1/(0.66)^2 gives the permittivity (assuming permeability of 1, which is very close in this case), about 2.3.  (The accepted value is 2.25, not bad, huh?)

How about twisted pair?  With ordinary insulated wires, you get a hybrid mode of field in dielectric and in air.  This gives a higher impedance (typically around 100 ohms), and a higher velocity factor, typically 0.70 to 0.85.


Heh, hmmm, so suppose you take a coax cable over a ground plane, drive one end's core with respect to GP (shield open), and measure what's on the other end.  The inside-coax VF is 0.66, while the outside VF should be >0.9.  This should give rise to the interesting feature that you can see a wave arrive before it should arrive.  I should demonstrate that, just to stab at the people who still believe you shouldn't ground both ends of a shield. 

Tim

[Beamin]

So the coax dielectric slows it down. But what if you're using coax to bare copper? Should your receive only antenna follow this rule? Is this only apply to transmitting and losses? I was under the thinking that anything that applies to tx also applies to rx like radiation patterns are the same. Slower means shorter antennas: the page says Frequency
433MHz
34.6cm   34.6 x 0.95 = 32.9cm
868MHz
17.3cm    17.3 x 0.95 = 16.4cm
915MHz
16.4cm     16.4 x 0.95 = 15.6cm
so all of my antenna are too long. Some youtube DIY totally ignore this fact.

So for coax to copper wire you calculate your feed line at one speed and your wire at another  The speeds don't average do they?

Why does 1/4 or 3/4 wave length feed line radiate lots of energy but not 1/2? When going longer then 1 wave length do you avoid 1 1/4 and 1 3/4? Does this become negligible at long lengths like 5 or 10 wavelengths?

[T3sl4co1l]

Speed is instantaneous.  It's just one-dimensional index of refraction.  When light passes through glass, its wavelength (as measured in a given medium) doesn't change just because it spent some time* in a medium with a different velocity.

(*Since photons have mass = 0 and velocity = c, they can't even experience time. From that frame of reference, generation and absorption happen in the same instant. )

You shouldn't need to calculate your feed line, but if you are using lengths of coax for matching, yes, the physical length differs -- by the ratio of velocity factors.  We generalize this by speaking in terms of electrical lengths: 1/4 wave might be 1m in coax, 1.5m in space, 1.2m in twisted pair, 1.4m in ladder line... or 10mm in a chunk of ferrite.  But regardless of the physical length, the electrical length -- some proportion of the wavelength, in whatever medium is being used -- is all that's important.

Antenna designs aren't much of a basis for comparison.  They suffer worse than appnotes -- the latter at least has to have some supporting data, and has the tacit approval of a big manufacturer.  Antennas can be literally a spool of wire and a pamphlet tossed into a box.  Almost no one bothers to measure them, and opinions consist more of superstition than science.

There are certainly practitioners of the craft who adhere to the scientific method.  If you see measured (not simulated -- but that, too, is at least a start) data, including feedline impedance or SWR, and radiation pattern and/or gain, you have found a trustworthy source.

The other catch is tuning: even a very poorly cut antenna can still be tuned to a desired frequency.  Sometimes depending on feed line length (which, length alone won't match everything -- you need two sections of different impedances and lengths to pull that off -- but it can convert an odd reactance into a resistance, or anywhere else around the circle of phase shifts), mostly depending on a tuner box (usually an LC or CLC network, something like that, with adjustable/selectable values).

And keep an eye out for unbalanced antennas.  There are a number of inordinately popular designs out there, that -- probably intentionally -- send signal current up the feedline.  They often make claims like, higher bandwidth or directivity than the overall dimensions of the antenna itself would suggest -- or can even permit** -- and customers often find inconsistent results (gee, I wonder why!) or dangerous results (large transmitter feedline currents, anyone?)!

**There is a physical limit relating gain * directivity * bandwidth, and the smallest enclosing sphere the antenna can fit within.

Anyway, back to theory: any odd multiple of 1/4 (i.e., 1/4, 3/4, 5/4 = 1 1/4, etc.) works as a whip antenna (giving an additional lobe in the radiation pattern as you go up for each mode), and any odd multiple of 1/2 (total length) works as a dipole.

Radiation is prevented (by interference) at even multiples -- though not totally, as the radiation resistance doesn't go perfectly to zero or infinity, but remains finite.  Mind you, the fact that it's very different from nominal impedance, means it's going to be much lossier to get there in the first place.  That is, most of the energy is trapped as reactive power, gobbling up losses but not accomplishing anything.

Or, I would arguably say that radiation does in fact go to zero***, except that no real antenna is perfectly balanced.  If you attempt to tune up a dipole at 1λ, what you're matching is not the radiating mode you thought, but something due to asymmetry in the antenna itself: the driven frequency not actually being perfectly harmonic, but differing a little from the antenna's natural antiresonance (and thus still having some radiation left to give); unequal lengths, or bends or kinks or bowing, in the elements; unequal proximity to nearby structures; imbalance in the balun, particularly feedline currents from a shitty type of balun like a current choke; etc. 

(***Unless I'm forgetting that a dipole at 1λ actually radiates just fine with axial lobes or something.  I don't think so, though?)

Tim

[SMdude]

How long is a piece of string?
What works for a good tx antenna, might not work as well for an rx.
And what works well for an rx, might not work well for tx.

I am working with some low power 2.4g transmitters and receivers and have found exactly that!
Luckily, my transmitter only transmits and the receiver only receives.
To tune my antenna lengths, for the tx I have done reflection measurements, finding the length that gives the least reflection then fine tuning when the antenna is installed.
I have tried this same method for the rx, however, what gives me good rx signal actually gives me quite poor reflection measurements, to my surprise!

The best way to set up an antenna(that I have found) is to first calculate and measure, then install and tune.
I have been using a LimeSDR to do this, which has given me much better results than stumbling around blindly.
I now have a spectrum analyzer on its way as this will(should!) make the process a bit easier and will also make it easier to properly characterize the antennas and see what effect the adjustments have across the full spectrum range. I can only see about 15mhz of bandwidth with the limeSDR(slow usb port on my computer).

Antennas are a bit of a black art IMHO.

[Yansi]

How long is a piece of string?
What works for a good tx antenna, might not work as well for an rx.
And what works well for an rx, might not work well for tx.

How does that happen, that properly tuned antenna does work for TX but not for RX (or vice versa)?

I don't understand that. Maybe, your TX and RX are not properly matched to 50 ohm (or whatever the antenna has).   You need a vector analyzer to determine the impedance, as the scalar measruement (only VSWR) is not fully characterizing the antenna, especially when there is a significant length of transmission line to it. I do not understand how a spectrum analyzer or SDR will help you do that. 

[T3sl4co1l]

Indeed, what works for rx, may not work for tx.  But anything that works for tx, works for rx.

The only exception is if your rx and tx aren't tuned properly.  If they are both tuned to best match, whatever that happens to be, then this remains true.

Most graphic example: AM BCB loop antenna.  Utterly terrible gain, -40dB or worse.  But for rx, it works as long as the noise floor, after the RF preamp, is below the noise floor of atmospheric noise.  Which is considerable at those frequencies, so you have a lot of leeway.  On tx, the gain leads to an utterly useless efficiency, even at the ideal impedance match.

Mind that integrated transmitters tend to be little more than CMOS transistors driving output pins.  This is at least true of most RFID interface chips.  It may be true of most BT chips (but using pretty small transistors so that the output power is small?), though it's probably not true of Wi-Fi chips (because they need a linear PA to handle all the modulations).

In any case, the small-signal impedance match is not always the best-power match.  This is most obvious on the linear amp, where the output resistance can be very high, much higher than the best-power load resistance.  In this case, a higher load resistance will exhibit more gain (more output power at low signal level), but cause clipping well below maximum power output.  Conversely, for the class-D output, the small-signal impedance match may be too much load for the transmitter, causing overheating or damage, and increasing THD/IMD.

Measuring the small-signal impedance of an rx or tx port may give incorrect results; the best you can do is measure the antenna port impedance, and match it to the manufacturer's recommendation.

Tim

[SMdude]

Yansi, it's an Anritsu S332D, which does have VNA functions, however, I am still going to have to learn how to drive this thing!

My coax lengths are all very short.

With the SDR, for the tx, I am using my actual transmitter for the signal gen and a return loss bridge and measuring the amplitude of the reflected signal.

For tuning the RX antenna I used the transmitter at a fixed distance from the rx antenna and sdr, and measured the signal amplitude, tuning first to receive the highest amplitude signal, and then trimming to have least effect when installed in its enclosure and finally with human body contact/proximity.

I guess the biggest thing with the poor performance of my reflection tests with the rx antenna, it its enclosure and proximity to everything, which is much different to the tx side. Ie, my rx enclosure is not a very good place to transmit from.

Perhaps I should have re-worded my statement. "What works for a good Rx antenna, might not work as well for a Tx."

Just for curiosity, with the 2.45ghz I am working with, I found the tx gave least reflection at 31.5mm(1/4 wave) and Rx best reception about 29mm.

I still have a lot to learn about antennas and the tuning of them, but I know a heck of a lot more about them than I did 6 months ago!

[Beamin]

So if I have this right: if I'm doing VHF/UHF better to have a few feet of feed line (75 Ohm coax in my case as I have tons of it) to make sure I'm far away from 1/4 wave length if I'm transmitting.

For the short wave HF Rx best to keep feed line(more 75 ohm CATV coax) as short as possible? Very general but it's easy to get lost in details.


When I was a kid and used 300 ohm latter line to feed a 1/4 wave 27MHz dipole in my attic I always confused on how the antenna knew when it was going from feed line to antenna. I had two: Both went to a
cobra 50 ohm CB cranked up to 8 or 10 watts
-> then through those little TV matching transformers with an F plug and two screws for latter line
->Probably 25' of feed line
->1/4 wave dipole that ran the length of my attic. __________
                                                                        ||
The cobra had a SWR metter that appeared when you transmitted and I got 5/5
There was a second antenna that was the same set up on it's own feedline perpendicular to the first but shapped like this:   /___\ due to the attic  not being wide enough.                          ||
That antenna got 3/5 SWR but people said I sounded better on it and could still easily key over almost anyone in the area. My house was on a large hill over looking the city so that helped alot. 
I could very easily key over people three towns away when they were within the same town (they were consideribly closer to each other.

EDIT: ASCII drawing is hard.
Still never figured out why this worked: the differance in feed line must have given the different SWR?

The antenna was just a 1/4 length ladder wire with the ends twisted together and one conductor cut in the middle to attach the two conductors of the feed line. I wonder if they still sell 300 ohm in stores anymore since radio shack is gone or if it' rediculously expensive now.