Copper Cactus

[Pro-95]

I've been doing a bit of reading on antennas. I found many references to the "Copper Cactus" or J-pole styled antenna. Seems as though this a popular and arguably easy to do, for the do it your-selfer antenna builder.

This antenna also seems to be able to produce some, albeit nominal, gain and if designd and cut for 144, 440, 900 Mhz could be a decent antenna. I found reference for even a 4 band J-pole.

I've also talked with a couple of people regarding antenna farms and they all seem to indicate that since there is no transmitting taking place that simply "T"ing connections should'nt have much effect of the listerners signals.

So has anyone built a copper cactus? Did you have much success with it? What about the "T" comment over duplex/diplex/multiplxing the signals to combine into one line down to a scanner?

 

[LarrySC]

Its a Ham antenna. Never heard of a scannist using it because its limited in band coverage. Scannist usually use tri-band antennas and/or broadband [no gain] discones.

 

[Voyager]

I've also talked with a couple of people regarding antenna farms and they all seem to indicate that since there is no transmitting taking place that simply "T"ing connections should'nt have much effect of the listerners signals.

They are absolutely wrong. Using a T will reduce each leg by 3 dB. Using three Ts will give you four connections, but EACH one will be down 6 dB if you connect them in the standard way.

1 2 3 4

. T T
. T

ANT

(OK, that didn't format well, but if you try to reply, it may show up in a better format)

If you use 2 Ts for 3 outputs, one (#3 in the diagram) will have 3dB of loss and the other 2 (#s 1 & 2) will have 6 dB of loss. (3 dB of loss from the first split, and another 3 dB from the second split)

.1 2
. 3
. T
. T

ANT

Anytime you split the signal without filtering (separating bands, for example), you will divide the signal between the two outputs.

Transmitting or receiving, the effect is exactly the same.

While we are at it, let's knock off another urban myth. The loss in a feedline will be exactly the same - transmitting OR receiving. IF the line has 3dB of loss for transmitting, it will have 3dB of loss for receiving in the same frequency band. Loss does increase with frequency. The same feedline that has 1 dB of loss at 150 MHz will typically have about 3 dB of loss at 450 MHz.

Joe M.

 

[Pro-95]

Its a Ham antenna. Never heard of a scannist using it because its limited in band coverage. Scannist usually use tri-band antennas and/or broadband [no gain] discones.

Ok now you've heard it .... *** J-pole for scanners design page *** ....

Here is a J-pole multi-band Here

Only given as examples as to why I thought that it would be ok. Seems that a small farm of these like 150Mhz, 400Mhz and 900Mhz should cover most scanning apps?????

But what do I know, that's why I asked the question.

 

[Pro-95]

Anytime you split the signal without filtering (separating bands, for example), you will divide the signal between the two outputs. Transmitting or receiving, the effect is exactly the same.

Thanks Joe, that is what I tell people when they buy "splitters" from me enough splitters and you don't have any signal. But like what I mentioned above, I am getting seemingly conflicting info, depending on whom you talk (read) to (from).

If you review the last J-pole link I posted the author indicates that multiple "T"'s for creating a multiband J-pole works.

 

[DaveNF2G]

If you use 2 Ts for 3 outputs, one (#3 in the diagram) will have 3dB of loss and the other 2 (#s 1 & 2) will have 6 dB of loss. (3 dB of loss from the first split, and another 3 dB from the second split)

The losses are cumulative. (Just expanding on what you said, not disagreeing.)

Starting from the antenna, the first T causes a 3dB loss, or in terms of power it cuts the signal in half. The second T causes an ADDITIONAL 3 dB loss relative to the first T, but the cumulative effect between the antenna and the receiver is that the signal at the receiver is only 1/4 as strong as it was at the antenna.

1/2 of 1/2 is 1/4. After four T-connections, the signal fed to the receiver is 1/2*1/2*1/2*1/2 of the original, or 1/16 as strong as the signal at the antenna.

This can be overcome in a couple of ways, each of which has disadvantages.

You could put a preamplifier at the antenna (the usually recommended point in a receiving system for such a device). The disadvantage is that interfering signals are amplified, too.

Alternatively, you could put the preamplifier at the receiver, where at least you can hope that only desired signals are still strong enough to be useful. The disadvantage here is that desired signals that were weak at the antenna probably aren't going to be present after all of the line losses, so there will be nothing to amplify.

73 de Dave, NF2G, NYSING-001

 

[N_Jay]

If you use 2 Ts for 3 outputs, one (#3 in the diagram) will have 3dB of loss and the other 2 (#s 1 & 2) will have 6 dB of loss. (3 dB of loss from the first split, and another 3 dB from the second split)

The losses are cumulative. (Just expanding on what you said, not disagreeing.)

Starting from the antenna, the first T causes a 3dB loss, or in terms of power it cuts the signal in half. The second T causes an ADDITIONAL 3 dB loss relative to the first T, but the cumulative effect between the antenna and the receiver is that the signal at the receiver is only 1/4 as strong as it was at the antenna.

1/2 of 1/2 is 1/4. After four T-connections, the signal fed to the receiver is 1/2*1/2*1/2*1/2 of the original, or 1/16 as strong as the signal at the antenna.

Not always (actually not usually true)

A splitter will have a theoretical 3B, typically 3.5 dB loss because it maintains the impeadance of the system.

A "T" will create an indeterminant amount of loss because it disrupts the constant impeadance of the transmission line and turnes the whole mess into a multipranch network witch would require extensive study to determiune the loss with any creditability. (anyone here actually USE a Smith Chart before?) :evil:

As for line loss it is equal in both directions but SYSTEM lost may not be due to many reasons such as reflections from various tuned elements and impeadance irregularities.

Like I have been saying, get the ARRL books and read. This is science!

 

[Pro-95]

Like I have been saying, get the ARRL books and read. This science!

I will be going to the local electronics store tomorrow as RS no longer carries ARRL books. :roll: Thanks!

 

[loumaag]

... (anyone here actually USE a Smith Chart before?) :evil: ...

:lol: Actually, yes. But not since computer modeling came into vogue!

 

[LarrySC]

WOW, so much detail. I'll stick with my tri-band with ONE connector. Good Luck.

 

[N_Jay]

... (anyone here actually USE a Smith Chart before?) :evil: ...

:lol: Actually, yes. But not since computer modeling came into vogue!

Yes, but using a SMith Chart will teach you how to look at a transmission line problem logically. It will help you assimilate the understanding into your "common sense".

The computer model will only give you an answer to an unreasonable (and not valid) number of decimal places, never providing you the logic tools to know if you asked the right question! :twisted: