Hi Doug,
Re:
" ... it seems to me that rather than embroiling a novice in toooo much technical stuff ..."
Fortunately or Unfortunately antennas like telescopes get rather complicated, so answering Barry's technical questions takes a technical answer.
Everyone was a novice once, you get interested and "learn by doing"
then one stops being a novice!!
Asrtonomy Antennas:
1. The Problem
The problem here is listening to "weekish" signals and designing the cheapest antenna that will function well enough to accomplish the end of recording a signal generated by Jupiter's storms and that fits in the allotted space.
From the web site previously mentioned the signal to noise ratio (SNR) of the signals are about 12db compared the signal to noise ratio required for a good picture (45 to 55 db). One gets these high SNR's ,50db, because most TV transmitters are very powerful (100KW) and/or in direct line of sight and high gain antennas are employed. Because of these conditions small mismatch in the antenna and receiver will not have "serious" consequences.
However, if the TV transmitted power were dropped to 100W (a 30 db drop in power) then condiderably more attention would have to be paid to the antenna and its matching to get a viewable picture.
We are now approaching the problem that Barry is faced with.
2. Random Wire
Use A random piece of wire?? Not really suitable here. Why?
Favours the wrong direction - pattern depends on wire length and is directed along the length of the wire. How long would it have to be? 1,2,3,4 or 5 wavelengths? Length could be from 14M to 70M ... too long.
How high? Zepp antennas are not noted for their efficiency!!
3. Mismatched dipole.
Well matched dipoles are just as easy to make as badly matched dipoles and are cheaper than buying an ATU. The cost of the dipole I am describing should be no more than $50 (with all new parts) and that includes the timber mounting posts and polyethlene rope guys compared to the price of an ATU only ($199). Construction and set-up time should be no more than 2 Hours.
4. Folded Dipole
Yes the feed point of a folded dipole in free space is 300 Ohms.
I did not mention a folded dipole because I had chosen to match this dipole via an adjustable T-match. This is a very simple match - very easily constructed, adjusted and cheap. One could use a gamma match (possibly better) but the construction is a little more tricky and one needs a variable capacitor - but no balun. chose a TV balun because it is easy to get one. One may construct a balun very simply but this takes a bit more experience and unnecessarily complicates things for now.
5. I have not measured a TV antenna but I have measured many antennas at antenna test ranges. You mentioned the resistance at the centre of a dipole (in a Yagi) as being 15 to 20 ohms - I'll buy for the time being.
But why would you want to feed the antenna from the centre? You could find a point away from the centre where the resistance is closer to 300 ohms.
The driven element of a yagi may have no break in it, 2 elements welded to the metal boom. The centre (at ground potential) is 0 ohms ie a short circuit!! his is the type of dipole I mentioned previously -a single wire dipole
fed symmetrically off centre.
6. Re: "If this is correct then using a 4:1 balun on a TV antenna would be very lossy". Yes, if you used the TV balun was connected to the centre of a dipole whoseresistance was 20 to 40 Ohma the output resistance would be 80 to 160 Ohms and the power loss would be very high due to a high VSWR and not lossed in the balun.
In all my posts I have avoided using the word impedance as that would
get too technical. For discussion of antenna matching in detail would involve
talking about the reactive and resistive components presented by the antenna and how the the matching section provides the conjugate match.
I only wanted to describe the construction of an antenna that is easy to construct and do the job, of course, also to answer a few questions.
G'say Jerry,
First of all let me try to clarify my mention of a random wire. I see from re reading my earlier post that one could fairly assume that I was promoting same. What was really back of my mind was the diversity of antenna that can be matched using an ATU rather than a good choice of antenna.
An ATU, for those unfamiliar with these things, is a device that at its simplest, requires no battery or power supply, it is merely a box with a few coils and a variable capacitor, an 'in' socket and an 'out' socket; it is used to present a 'good look' for a receiver/transmitter and at the same time a 'good look' for an antenna.( a master of negotiation )
I guess I should say at this point that I personally have about as much or less interest in Radio Astronomy as I do in knitting a jumper out of spaghetti, so if my lack of enthusiasm shows through, please don't be discouraged from trying anyway.
Jerry, I think we are looking at this from different perspectives.
To me, when someone says they are dealing with a 12db s/n ratio, I have a mental picture of a very weak carrier wave burried under a pile of QRN/QRM.
I think of front to back and side lobe rejection and capture area, forward gain. I think of being able to switch in a narrow band pass filter etc;
not a dipole.
But with static, and lets remember Radio Astronomy listens to static, not ET phoning home (sorry Seti ). Any simple low gain antenna will be like looking at the sky through a piece of ground glass. You can see that there is light if the sun or moon is up, but have little idea of direction or anything else about that light.
It is all very well to talk about VSWR, but without a transmitter and a bridge I'd think the concept purely accademic.
To get the optimum match with an antenna, say on the 15M band using a receiver only, we could tune to a moderate to weak nearby station and tweak the matching section till we get max signal strength, and as long as there is little or no fading we might achieve reasonably good results.
I question the practicality of this method when listening to static, and even by tuning to a nearby carrier signal, because one will still be plagued with QSB.(fade)
That is why I think an ATU would be the way to go, because without leaving the receiver, one could quickly (measured in mere seconds) tweak for loadest noise or highest S meter readings without much risk of confusion caused by the variability of the ionoshere.(fade)
If we were matching to an antenna usung a transmitter and VSWR, we could easily see it we were moving toward or away from match.
Count the equipment cost to do the same thing for a (receiver only) antenna set up. How does that compare to the cost of a basic ATU?
BTW a "match box" used for CB might only be $50 or so from Dicksmith etc.
But not as good as the $189 job......you only get what you pay for.
Sorry Baz. Jerry and I aren't slugging it out; there is no conflict, no arguement, no contest. I thought you were serious about wanting to listen to noise originating from places other than planet Earth. I'm sure we both wish you well in your project.
Jerry, you can use the "manage Attachments button when you post as usual. Otherwise, there is an email on the first page of my website that you can send it to.
Doug, no sweat mate, I just found the exchange of expertise between you two, amusing at times and akin to a "one-up" on knowledge!
Don't get me wrong mate, I am taking notes because even though I don't know what you are talking about sometimes, I recognise that it is gold material and I am looking it up.
I just got a book from the library yesterday, Titled, "Electronics, the easy way." It has a bit about radios and antennas in it, but also gets back to basics of ohms, volts, impedance and stuff like that which, if youre like me, you have no idea about. This becomes a serious set of brakes when one is trying to become an amateur radio astronomer.
But with static, and lets remember Radio Astronomy listens to static, not ET phoning home (sorry Seti ). Any simple low gain antenna will be like looking at the sky through a piece of ground glass. You can see that there is light if the sun or moon is up, but have little idea of direction or anything else about that light.
I think this is a very important point to realise before making any significant investment in this project.
How can you know the source of the noise you are receiving unless it can be verified by steering the antenna toward and away from the source?
The antenna design on the first page of this thread looks like a directional design, but its physical size looks to be very small for an antenna designed for 21MHz, and hence will have a small capture area. This means a low efficiency at plucking signal from the air and you would find that a simple dipole will pick up much more signal, but then a dipole isnt steerable
Antennas with usable directionality and gain become much more managable at higher frequencies due to their smaller size.
Thanks Starkler. I agree that the antenna is too small. I got the measurements from another site, but it looks like the bloke held it by hand . Mine I want to stay mounted, so I think I may make a bigger one, along with a few other antenna experiments, like, quad, helical, yagi and dipole. I have to watch the size though sa the neighbourhood would probably not appreciate the clutter.
Re: Capture area is a function of wavelength that is why physically small HF antennas work as well as large antennas. The main issue with small HF antennas is the very low radiation resistance. For these small antennas to work efficiently the "ohmic losses" must be held much lower than the radiation resistance hence the thick conductors - much greater attention must be paid to construction. Smaller antennas have a major drawback in that they have mucher lower bandwidth (high Q) and must be tuned to each freq. for best performance.
eg. A half wavelength vertical has a radiation resistance of many thousands of ohms a short vertical (DDRR antenna) may be 0.1 Ohms. Both these will perform the same except that the half wavelength antenna can be made much more efficient than the short antenna.
OK on the choice of antennas except that I would steer well clear of helical antennas at the frequency in question. The helical is a wonderful antenna at 400MHz and above where you would normally use them in groups of 2, 4, 8 etc. to act as 1 hi-gain antenna.
Suggestion: As you have increased the range of antennas to choose from and noting that the signals you are interested in go upto 40MHz. You may consider a log periodic antenna that will operate on all freqs from 20 to 40MHz.
But why would you want to feed the antenna from the centre? You could find a point away from the centre where the resistance is closer to 300 ohms.
Well 75 ohms anyway.
Interesting observation Jerry, and were it a conventional Yagi I would agree without much reservation. A TV Yagi (I think) is a bit of a "this goes with that" array. (in no sence is it log periodic) Impedence would be anything but flat over the bandwidth in use. So strictly speaking a good match is only a pipe dream no matter what you do unless you want to switch in tuned matching sections for each station. Frankly program content is not worth the effort.
Cheers,
Doug
Yes Doug programme content in some cases demands a vertical or no antenna at all - the viewing is better under these condx.
Re: Helical I mentioned to steer clear of - I left out an important part that opens the way to argument by some knowledgable people - well I'll close the gate now. Due to misunderstanding on my part I assumed an axial mode helix but one may attempt to use a broadside helix - I'd siill steer clear of these !!
I have designed, built and tested (not mounted - wife thinks they look awful!) a 15 turn helical for UHF TV (bandwidth 0f 250 MHz - almost flat gain 13 db) - The perfect antenna for uhf TV.
I could design one for you but the construction is a bit more complex - not difficult but "fiddly". One would make it out of wire but the problem would be mounting it pointing upwards - Easily done but it would end up looking like a sailing ship's rigging.
The log periodic is a broadband antenna with a gain equal (about) to a dipole where the resonance moves smoothly from one element pair to the next. They are mostly used for receiving a wide range of signals - say using a scanner . I have built only one "serious" LP in my life it was for a job and was a "*****" to build setup and test - brief spec - LH circularly polarised LP flat from 10MHz. to 1000MHz.
Jerry, I had a go at the dipole antenna from the website you suggested.
What's confusing me still is how one arrives at the cut lengths for each side of the dipole.
The calc on the site 467 divided by freq, then halved, arrives close to the same result i get from my calc of 300 divided by freq, then halved and halved again. The end result is a difference of quite a few inches. What I don't understand is why the difference? which way is the right way?
The virus checker does not pass pics to me - please e-mail them to me, thanks very much.
OK on the difference in the two calculations.
The difference may be explained by the speed of the radio wave in space and "on the wire".
1. The speed (velocity) of the radio wave is
Velocity = Frequency x Wavelength
in space the velocity is 300,000,000 metres per second - a closer number is
299,700,000 metres per second.
Wavelength = Velocity / frequency
The main thing is that this velocity changes with the medium in which the wave is travelling. In space it is 300,000,000 M per sec (light speed!) in air it is slightly slower and on the wire the speed is about 0.98 times 300,000,000 metres per sec.
The other factor that "mudies the water" is the thickness of the wire compared to the wavelength and as the wire gets thicker the length of the antenna should get shorter.
So to be accurate you use a velocity of 282,000,000 metres per sec. (ie. 94% of 300,000,000) as the velocity and not 300,000,000.
Notice all the zeroes every where ok?
Now to calculate the wavelength at a frequency in Mhz (notice 6 zeroes in the top line and 6 zeroes in the bottom line disappear when
wavelength(metres) = 282/freq (MHz.)
For 21.9MHz, wavelength = 282/21.9 = 12.87M divide by 2 for a dipole and you get 6.44M.
The number 467 is usually found in Yankee books- if you divide 467 by freq in MHz you get a length in feet. So using your number 467/21.9 = 21.32 feet = 6.5M close to our guess!! Their number assumes different wire to what I've chosen - use their formula.
Jerry is correct. I did some approximate calculations last night and came up with a difference of 6mm per leg of the dipole - depending on whether I used 186,000 miles per second or 300,000 Km per second as the speed of light. Both of these are approximations. Add in a little rounding of figures in my calculations and they are both 'near enough'.
You, in this case, are dealing with a free space wave length of around 20 metres. .006 of a metre will make no discernable difference to your reception (nor would, probably, a foot or 300mm to be honest). It's not as though you are trying to grab a narrow band signal - which you won't do with your receiver anyway, as it's designed as a broadband unit. So don't worry too much, and certainly not to millimetre accuracy, about the length of the dipole's legs. With what you are attempting I would consider 'close enough is good enough'.
Now, your receiver:
I am not familiar with the Kenwood unit you have to hand, but it appears that the various controls in your third picture, bottom row are as follows;
The group of four 'function' buttons - from the left
AM - this is amplitude modulation and is the same as the AM broadcast reception on a home radio. This method recognises the peaks and troughs of a signal in order to produce noise. It usually relies on the transmitting station sending a 'carrier' on which the modulation is imposed.
AM ANL - this looks like a noise limiting function. Most of these are set to take off (or filter) the short duration spikes of a received signal. If it works properly it will dampen such things as local power line interference and noise generated by such things as electric motors.
CW SSB - this mode is similar to AM but uses only part of the received signal. In effect it looks at a narrower band width than does AM and uses either lower or upper methods. I don't know which your unit uses as it doesn't stipulate. With what you are doing this probably doesn't matter. This, by the way, does not need the 'carrier' used by an AM transmitter.
FM BC - I'm assuming that this is FM (like the FM stations on your home radio). This, instead of using the amplitude of the signal to sort out the message, uses the change in frequency of a signal to produce the output you hear.
FM AFC - This is a guess, but I think this is simply a function enabling the radio to 'lock on' to an FM signal and, within reasonable parameters, keep the radio tuned to the signal being received.
Now the others:
It looks to me as though the RF marker is designed to generate a low level signal (internally) to enable calibration of the frequency dials. I am wide open to correction on this one.
The band selector, I would think, is reasonably self explanatory.
The antenna (aerial) trimmer is provided to 'peak' received signals. This is very handy when using the same piece of wire for various signals. It's a crude, but effective, matching unit.
AF - this is simply the audio frequency gain control - what most of us call a volume knob.
I hope that lot helps.
Ah, yes, the dual tuning drums.
Again I must emphasise that I am not familiar with this receiver.
The drums though could be a coarse tuner (the top one) and what is sometimes called a 'band spread' tuner (the bottom one). This enables you to set the upper drum to the frequency you are after and then fine tune using the lower one.
*edit*
I should add that I'm not too sure what you are going to gain out of the exercise. Have fun anyway.
I don't know that particular Kenwood either.
The RF marker, might be a novel name for a BFO. I think one would be highly usefull since it seems not to switch from USB to LSB. Would be of enormous help weith CW too. But I wouldn't put money on it.
Doug
). Any simple low gain antenna will be like looking at the sky through a piece of ground glass. You can see that there is light if the sun or moon is up, but have little idea of direction or anything else about that light.
LOL
