Which is a better design. Suppose you have a 6 inch length of PVC pipe with numerous turns of wire that has an inductance of say 200uH. Now suppose you use the same (6 inch) piece of PVC with a ferrite rod in the core with considerably fewer turns of wire. Which one would capture the most signal at the AM Broadcast frequencies (500K to 2 Megs) and poduce the greatest signal output? Would it be more ferrite, or more wire?
Considering that all portable transistor radios I've ever seen had ferrite core antennas I would assume (not have the math) that ferrite is more efficient in small spaces. I don't think the price of the copper wire back in the 50s and 60s would make enough difference to prefer ferrite over copper wire...
John
** Your Q is stupid, ferrite antennas must have the wire wound on the rod.
Woven, frame antennas like this were used in compact and portable tube radios until the arrival of ferrite rods.
Their performance was comparable with a ferrite rod antennas of similar length - the ferrite job having rather higher Q due to using less wire.
Most frame anttennas use "Litz" wire to improve the Q - as did ferrite equivalents.
FYI
Q = ratio of resistance ( at radio frequencies) to inductance.
.... Phil
You are asking how the antenna output voltage varies with the mu of the inductor core if the inductance is held constant by varying the number of turns of wire.
The signal strength is related to number of turns linearly as well as the core permeability. I'll use the tilda to indicate proportionality.
E ~ N * mu
Inductance is proportional to the square of the number of turns and the permeability.
L ~ N^2 * mu
So when a magnetic core is added the number of turns required to maintain the inductance is reduced by the square root of the relative permeability.
N ~ sqrt(mu(rel))
The impact on the received voltage will be reduced by the square root of the relative permeability through the turns change, but increased linearly by the change in mu resulting in an overall increase in received voltage by the square root of the relative permeability.
So adding a material with mu(rel) of 4.0 will cut the number of turns in half and increase the output voltage by 2. This doesn't include an improvement in the resistive losses of the coil. The Q of the coil will be reduced as the inductance drops linearly, but the resistance goes down by the square root of the permeability.
-- Rick C
And in the AM broadcast band, the SNR is usually not determined by the radi o but rather by the atmospheric noise and interference form other stations, ,,, so even though you may build an antenna that puts out more "volts" i t will not be helpful for picking up weaker signals.
M
It is much more likely that higher inductance is needed at AM broadcast frequencies in order to keep the tuning capacitaance at a reasonably low value and because of limited space in a portable or tabletop radio.
Dave M
A ferrite rod sucks up and concentrates the H field. More flux passes through the core than if the same geometry were all air. And it needs a lot less copper to sample that field, an to make a nice inductance to resonate.
In your case, use a smaller winding directly on the rod or (harder to do) fill the entire insides of the tube with ferrite.
The rod inserted into your pipe and winding will increase the flux, but it's not optimum. The people who have been making radios for close to a century have probably optimized the design.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Optimized for cost to sales ratio, certainly, but maybe not for the best performance to size ratio.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
Well, I can't help it if I'm stupid. I was just born that way. And I did assume the wire should be wound around the ferrite rod usually using Litz wire to reduce the skin effect. But there are some eddy current losses in the ferrite and I was just wondering if you knew how much that woud be?
Yes, that's true. It really doesn't matter what the signal strength is, only how much additional noise you introduce into the system.
So, I think you are saying adding more ferrite into the core and reducing turns of wire has some signal level advantage? But considering SNR, it doesn't much matter?
If you are building a coil to a specific inductance, then yes, you get more signal with a ferrite core but the difference is not a lot.
The issue of SNR depends on many factors. You still need a low noise first amplifier to raise a small signal from an antenna. The point is that if you have an adequately large signal from the antenna already, a lower noise in the amp won't be useful as the noise in the signal will be much greater. But that depends on the specifics of your signal field strength, the signal SNR, the effective height of the antenna (how well it converts the field strength to volts) and the noise level of your receiver front end. How many of these do you know?
BTW, the coil does not need to be wound literally on the core. I've seen many antennas that were wound on a cardboard or plastic tube with the ferrite not tight inside. I can only assume they did that to make the manufacturing easier.
-- Rick C
** But you can piss off and stop being a PITA troll. ** No excuse for you to go around annoying people.
** No way anyone can know what you claim you assumed but did not post.
And you can make up *any damn shit* later.
** That is nothing like your original Q and no question was directed at me.You do NOT direct questions at posters except to explain what they have posted. All other question are directed to the group.
You are making shit up, moment to moment.
Kindly f*ck off.
.... Phil
** Providing there is an adequate signal from the antenna carrying the wanted broadcast. ** Complete non-sequitur and obviously false.
..... Phil
** Most AM sets have an antenna coil or tuned transformer for that.
A ferrite or woven frame antenna is tuned directly by the tuning gang.
.... Phil
** The latter is exactly what a ferrite antenna is optimised for.
.... Phil
** Wrong assumption.
If you knew a damn thing about AM radios, you would know that the coil is made *movable* on the rod so it's inductance can be adjusted to peak resonance with the frequency being received.
Normally, you do this at the bottom of the band and use a trimmer cap for the top end. The coil is then held in place with melted wax or similar.
.... Phil
In the early days of transistor radios, size was limited and gain was expensive, so it was worth some ferrite to get more RF input power. Gain is now so cheap that an air core antenna might be OK.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
** Really ? Try posting an idea that is not full of ambiguities.
... Phil
It seems to be more ferrite. The latest and greatest ferrite antenna is the Ferrite Sleeve Loop (FSL). It uses multiple rods, or bars arranged side by side in a circle with wire wrapped around it. Diameters range from 5" to 15" although one day someone will spend the money to make 20" Here's one that that is 7" in diameter and uses 68-5/16" diameter rods
5" in length.Optimization of Ferrite Sleeve Antenna.
Lots of comparison data here.
For determination of the design factors influencing an FSL antenna?s weak-signal performance, four of the FSL test model match ups were considered very important. These four match ups (with the design factors clarified by the results) were as follows:
Followup on the link.
Search FSL for more info, Gary Debock is an experimenting guru. Mike
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