best antenna

[trigger]

this aerial here, i have just made i am assuming that one is connected to the shield of the coax? To get any real analyser results it will have to wait until Friday, cant get this one on me bicycle

I believe the core of the coax is connected to the top half and the braid to the bottom half. I made one of these but didn't find it particularly good.

[SpaxmoidJAm]

That is how i read it,so far its not looking to bad. Still haven't made anything yet that has beaten my one from page 79. at this rate its my recommendation as the one to make, made one for someone else didn't get time to put it on the analyser to see if it needed tuning and he's getting 150NM from his loft.

I still want to make a collinear that is actually good.

[abcd567]

this aerial here, i have just made i am assuming that one is connected to the shield of the coax? To get any real analyser results it will have to wait until Friday, cant get this one on me bicycle

The antenna is to be cut in the middle (at the feed point shown in drawing). Cable core is connected to upper half, braid to lower half.

[abcd567]

@abcd567
on page 20 of this document it has a slightly different arrangement could you simulate it and see if it wrong or right.
http://www.146970.com/PDFs/Antenna%2...na%20Ideas.pdf

@SpaxmoidJAm
Below are two simulation results
(1) Screenshot 1 - Franklin with 1/2 wavelength matching stub (as shown on page 20 of document)
(2) Screenshot 2 - Franklin with 1/4 wavelength matching stub.

SCREENSHOT 1 - FRANKLIN, 4 ELEMENT, WITH HALF WAVELENGTH MATCHING STUB
Franklin 4 element halfwave matching stub.png


SCREENSHOT 2 - FRANKLIN, 4 ELEMENT, WITH QUARTER WAVELENGTH MATCHING STUB
Franklin 4 element quarter-wave matching stub.png

[SpaxmoidJAm]

the 1/4 looks marginally better, still not much in it, i guess you could probably never tell in real life.

[abcd567]

the 1/4 looks marginally better, still not much in it, i guess you could probably never tell in real life.

The gain of 1/4 is marginally better, and SWR of 1/2 is marginally better.
One advantage of 1/4 is that it has a shorter tail (matching stub).

.

[lutorm]

Comparison of simulation results shows that 7.5 element shorted top is slightly better than 8 element open top. Please see screenshots below.

SCREENSHOT 1 of 2 - Seven-and-Half (7.5) Element CoCo - SHORTED TOP
Gain = 9.23 dBi, SWR = 3.87 (mismatch loss = 1.85 dB)
Net Gain Seen by Receiver = Antenna Gain - Mismatch Loss = 9.23 - 1.85 = 7.38 dBi

[ATTACH=CONFIG]3898[/ATTACH]


SCREENSHOT 2 of 2 - Eight (8) Element CoCo - Open Top
Gain = 9.52 dBi, SWR = 4.63 (mismatch loss = 2.33 dB)
Net Gain Seen by Receiver = Antenna Gain - Mismatch Loss = 9.52 - 2.33 = 7.19 dBi

[ATTACH=CONFIG]3899[/ATTACH]

I've been checking out your antenna calculations with interest. Nice work on trying the different combinations. A question though: in your calculations here, are you referring SWR to 50 or 75 ohm? Since the screenshot says "SWR 50", it sounds like they are referring to 50ohm (which would make sense if the program is for amateur radio)? If so, hooking it up to 75ohm coax would give a different SWR.

Also, how are you calculating the mismatch loss? It is my understanding that an impendance mismatch causes reflections, but the reflected waves will reflect back again at the back end of the antenna to the receiver. Mismatch will only cause additional loss due to the fact that we have higher power as a standing wave in the feedline compared to what we would have in the case of a perfect impedance match. I'm not at all an expert, but this is what I got out of e.g. http://www.arrl.org/files/file/Techn...f/q1106037.pdf. Most examples talk about transmitters, but I assume a receiver works in fundamentally the same way.

Cheers,

/Patrik J / KB1VGP

[abcd567]

I've been checking out your antenna calculations with interest. Nice work on trying the different combinations. A question though: in your calculations here, are you referring SWR to 50 or 75 ohm? Since the screenshot says "SWR 50", it sounds like they are referring to 50ohm (which would make sense if the program is for amateur radio)? If so, hooking it up to 75ohm coax would give a different SWR.........

/Patrik J / KB1VGP

Gains (dB) are same for 50 ohm & 75 ohm. The difference of SWR for 50 ohm & 75 ohm is marginal. Here the purpose was comparision, and result of comparision is same weather done on 50 ohm system or 75 ohm system.

[abcd567]

I've been checking out your antenna calculations with interest................Also, how are you calculating the mismatch loss? It is my understanding that an impendance mismatch causes reflections, but the reflected waves will reflect back again at the back end of the antenna to the receiver. Mismatch will only cause additional loss due to the fact that we have higher power as a standing wave in the feedline compared to what we would have in the case of a perfect impedance match. I'm not at all an expert, but this is what I got out of e.g. http://www.arrl.org/files/file/Techn...f/q1106037.pdf. Most examples talk about transmitters, but I assume a receiver works in fundamentally the same way.

Cheers,

/Patrik J / KB1VGP

Please read this article in wikipedia
http://en.m.wikipedia.org/wiki/Mismatch_loss
It will answer most of your questions. If still some of your questions are unanswered, please feel free to ask in this forum. I will try to answer unclarified points.
Best Regards
abcd

[lutorm]

Please read this article in wikipedia
http://en.m.wikipedia.org/wiki/Mismatch_loss
It will answer most of your questions. If still some of your questions are unanswered, please feel free to ask in this forum. I will try to answer unclarified points.
Best Regards
abcd

The formula in that page describes the relation between incoming, transmitted and reflected power. But the argument made in the article I linked is that the reflected power doesn't just go away, it reflects back and then reflects back again (at the transmitter, in that article) and comes back to the place where we calculated the mismatch. So the incoming power is really higher than you would think, because it consists of both the "primary" power and the part of the reflection that makes it one round-trip, and then the part of that reflection that makes it another round trip, etc. Thus, if you have completely lossless feedlines, there will be no loss regardless of SWR. If there is loss in the feedlines, a higher SWR will cause higher loss, because part of the reflected power gets lost as well, but it will only be as large as the formula in the Wikipedia article implies if the feedline losses are very large. (Look at Fig 1 and the reasoning in the text behind it.)

Additionally, I think we have to distinguish two different regimes when receiving:

One is where the signal is extremely weak and drops below the sensitivity of the receiver at maximum gain. Because antenna SWR controls the amount of power intercepted, a higher SWR will cause less power transmitted on the feedline and since the receiver is already at max gain, this will hurt. In this case, adding an amplifier will also obviously help.

In the other regime, though, the receiver gain is not maxed out, so antenna power isn't the limiting factor, but signal/noise is. In this case, SWR doesn't matter at all, because it will attenuate the signal and the noise picked up by the antenna by the same amount. Instead, the S/N will be determined by the directional pattern of the antenna and how much of the signal is picked up in relation to how much noise is picked up. Noise sources are things like the human EM noise and astronomical radio sources. See http://www.w1ghz.org/noise/noise99.pdf and http://en.wikipedia.org/wiki/Antenna...se-temperature. http://www.qsl.net/yu1aw/Misc/vhfnoisetemp.pdf seems to indicate that at 1GHz, man-made noise dominates with a noise temperature of 300K to 100,000K depending on location, approaching but still much higher than the noise of typical receivers, especially in urban settings.

If this is true, it would mean that SWR would be less important (especially when using a preamp) than having an antenna pattern that minimizes antenna noise temperature. The last link explicitly says

From diagram which is given on Fig. 6, it is obvious that gain of the antenna is the most important factor for good S/N on VHF and lower UHF bands for both terrestrial and space communications. But it is also obvious that for determining the S/N for space communications on frequencies above 1GHz, the noise temperature of antenna becomes as important as the gain of the antenna.

Many things to think about... I've had an amateur radio license for 20 years and know a lot about electromagnetic radiation, but only in an astronomical context -- I've never seriously tried to understand antennas.

[abcd567]

The formula in that page describes the relation between incoming, transmitted and reflected power. But the argument made in the article I linked is that the reflected power doesn't just go away, it reflects back and then reflects back again (at the transmitter, in that article) and comes back to the place where we calculated the mismatch. So the incoming power is really higher than you would think, because it consists of both the "primary" power and the part of the reflection that makes it one round-trip, and then the part of that reflection that makes it another round trip, etc. Thus, if you have completely lossless feedlines, there will be no loss regardless of SWR............

The terms "Reflection" & "Loss" here do not carry the literary meaning. These are used to create an easy visual picture. There is no wastage of power implied by "loss" (like wastage of power in a resistor in the form of heat). What it means is a lack of 100% transfer. It is ratio of delivered power to available power. Reflection also implies the same thing. There is no actual reflection of power like light reflected from a mirror. These terms are used to make them easy to underdtand by analogy.

[lutorm]

I found this page that calculates gain/temp for a system: http://www.satsig.net/noise.htm

I guesstimated some numbers: For the LNB, something like a typical cable amp that people have been using here, 15dB gain, and a noise figure NF of 6bB (I found a page that said their high quality cable amps had NF of 3dB, so twice that fore some cheap ebay amp seems reasonable.) For cable loss, 10dB @ 290K, and then a receiver NF of 3.5dB from http://superkuh.com/rtlsdr.html.

With these settings, you get a system noise temp of 1250K with 0K antenna temp (pretty equally contributed by the amp, the cable, and the receiver.) If the antenna temp is higher than that, it will dominate the system noise, and since the link in my previous post seems to indicate that the antenna temp can easily be 10x or 100x higher, the G/T of the antenna will determine the total noise figure.

I wonder if there's some way we can estimate the antenna temperature from your antenna calculations...

[lutorm]

The terms "Reflection" & "Loss" here do not carry the literary meaning. These are used to create an easy visual picture. There is no wastage of power implied by "loss" (like wastage of power in a resistor in the form of heat). What it means is a lack of 100% transfer. It is ratio of delivered power to available power. Reflection also implies the same thing. There is no actual reflection of power like light reflected from a mirror. These terms are used to make them easy to underdtand by analogy.

I disagree. There's definitely a flow of power in the feedline which reflects at the interface. (Hence the name SWR, standing wave ratio - the standing wave is made up by the two waves propagating in opposite directions.) And the loss I'm talking about (in the feedline, not the mismatch loss given by that formula) is (partially) caused by current turning into heat due to the resistance of the wire and the capacitance of the dielectric (and partially by radiated loss, as well).

[abcd567]

I disagree. There's definitely a flow of power in the feedline which reflects at the interface. (Hence the name SWR, standing wave ratio - the standing wave is made up by the two waves propagating in opposite directions.) And the loss I'm talking about (in the feedline, not the mismatch loss given by that formula) is (partially) caused by current turning into heat due to the resistance of the wire and the capacitance of the dielectric (and partially by radiated loss, as well).

It is Voltage wave which is reflected: VSWR "Voltage Standing Wave Ratio", not "Power Standing Wave Ratio".
What about DC, where there is no wave, no reflected wave, no standing wave, and still full power does not flow from source to load until impedance matching is done i.e. source resistance is equal to load resistance (in DC circuits impedance = resistance)

[trigger]

Finished my testing with the 3 element CoCos to try to work out the Velocity Factor of the coax I have. I've settled on an element length of 94mm which gives a velocity factor of 68%.

I've just, very carefully, made an 8 element CoCo with open top and put it on test. The first 20 minutes suggest it is as good as the Franklin I am using as my "one to beat" Will post graphs later after the thunderstorm moves through!

Cheers
Dave

As promised
8 segment coco.JPG31 March Franklin.JPG