800 ohm transmission line from Ethernet cable?

[Swamp Thing]

If the 8 pairs in a CAT-5 LAN cable are wired as shown so that their impedances add up, will it work well as an 800 ohm line up to around 30 MHz?

.... Or, would I have to slit the plastic sheath and put some separation between the pairs?

 

[Baluncore]

8 pairs of 100 ohm, will make an 800 ohm cable in theory, but I cannot see how you could benefit from that arrangement.
Also, I thought that cat 5 cable usually had 8 wires, twisted in four pairs.

Normally, you would drive each pair with a differential driver, to avoid common mode signals, but you cannot do that with the lines fed in series. It is a waste of pairs, since 100 ohms balanced, is easier to drive than a higher impedance, needing a higher voltage.

 

[Swamp Thing]

Oops, brain glitch. Yes, that's 4 pairs, so 400 ohms.

The idea is to make a feed line for a shortish ( $l << \lambda / 4$ ) receiving antenna for 1 to 30 MHz. The receiver is a portable broadcast rx with a built-in whip and no antenna socket. In this case the antenna and the Rx would be quite high impedances, so I'm thinking that a higher impedance feedline would mess things up less than a 50 ohm one. I will be clipping one side of the feedline to the rx's whip, and connecting the other side of the feedline to the earphone jack's ground. Not aiming for a perfect match, just qualitatively best solution.

If this overloads the receiver I will either shorten the antenna or just wrap the feed wire's end around the rx whip for adjustable capacitive coupling.

So question is, would the 400 ohm (or even 800, using two LAN cables) be noticeably better than a 50 ohm feed line? (Feed line length is around 3 meters).

The rationale for using the feedline etc is to put most of the actively receiving part out of the room to minimize noise pickup from within and to avoid the shielding effect of the walls.. Also, I notice that the reciever picks up a stronger signal if held in the hand, as opposed to placed on a table. I tried attaching a wire to the earphone ground, and that helped even more than holding the rx. But at certain times of the day the added couterpoise also picks up noise (which is a function of the way the counterpoise wire is arranged) -- hence the attempt to put the actual antenna outside in a balanced arrangement.

 

[Baluncore]

The receiver is a portable broadcast rx with a built-in whip and no antenna socket.

My guess is the receiver has an internal ferrite rod antenna, which is equivalent to a small magnetic-field loop antenna, orientation dependent. The built-in whip is an electric-field sensing antenna, which works best when you stand the receiver on the ground with the whip extended upwards. The two antenna modes, electric and magnetic, are coupled in quadrature by a very small capacitor in series with the whip. Before going further, trace out the front-end circuit used by the rx.

You could wind a few turns of wire around the ferrite rod, or around the whole rx, to couple in a lower impedance twin feedline. Minimise the electric-switching house-noise picked-up by the whip, by not extending it, or by internally disconnecting it.

In these days of switching power supplies and LED lamps, I would use an external loop antenna, with a twin feedline into the house. I would have a variable capacitor at the rx, in parallel with the feedline, to tune out the line and loop inductance. Wind the number of turns on the external loop antenna, to get the inductance resonant with the variable capacitor.

 

[tech99]

(i) The ferrite rod is usually used for the AM band only. (ii) The twisted pair cable you suggest relies on being balanced to reduce interference and that will not be the case. (iii) The new antenna might have 5pF capacitance and the cable might have 20pF so this reduces the voltage by a factor of 0.2. (iv) Use a counterpoise or ground plane wire at the remote antenna, connected to coaxial shield (v) One approach is to use coaxial cable but with a step down transformer at the antenna and a step up transformer at the receiver, using ferrite rings. (vi) Obviously the bigger the remote antenna and counterpoise the better, assuming it is away from interfering sources. If you have a location suitable for a large antenna it might be possible to use it with a coaxial cable without any transformers. I suggest carrying the receiver to new positions to find a quiet location for the new antenna.

 

[Tom.G]

8 pairs of 100 ohm, will make an 800 ohm cable in theory,

Uhmm... What I see in the OP's drawing is seven Gimmick capacitors in series between the antenna and receiver, definitely not an impedance transformer! See posts #8 (by @Baluncore), and #9. The rest of this post is valid but irrelevant.

Please note that the RF impedance of a transmission line is an inherent characteristic describing the Voltage/Current ratio when driven with a given power.

For twin-lead or coaxial cable, the Characteristic Impedance is a function of conductor diameter, conductor spacing, and the dielectric constant of the material between the conductors. Putting 8 transmission lines in series does not change the impedance they present to the signal source, it just makes a longer transmission line.

A single wire between an antenna and receiver is conceptually just an extension of the antenna, but since it is generally near other objects like trees and walls of a building its impedance is generally varying and indeterminate. As a vague rule-of-thumb, if the length of the downlead is less than 1/10 wavelength you can ignore the proximity effects from nearby objects.

Some of the active Ham Radio operators here can probably fill in the details much better than I can; it has been too long since I've dealt with the subject.

Much detail is available in "The Radio Amateurs Handbook" published by the American Radio Relay League (ARRL), and in "Reference Data for Radio Engineers: Radio, Electronics, Computers, and Communications" published by Howard W. Sams & Co. Inc.

Cheers,
Tom

 

[tech99]

For purposes of illustration, if each pair is terminated in a 100 Ohm resistor, then the input impedance of each pair will be also be 100 Ohms. If we place eight of these in series, the total input resistance will then be 800 Ohms. Similarly at the sending end.

 

[Baluncore]

Uhmm... What I see in the OP's drawing is seven Gimmick capacitors in series between the antenna and receiver, definitely not an impedance transformer!

It is not supposed to be an impedance transformer. It is a transmission line.
Except, if you look closely, you may notice that pair #2 is a transmission line inverter.

Consider eight transmission lines with characteristic impedance 100 ohms. Terminate each with 100 ohms. There will be no reflection from the terminations, so length is unimportant. Now connect the eight input pairs in series, to get an input impedance of 800 ohms.
If all lines have the same electrical length, you can remove the termination resistors and connect the output pairs in series, to get an 800 ohm output impedance, but only if the input is driven by an 800 ohm source.

 

[Tom.G]

OK.
Thanks for the clarification. I had missed that inversion on the 2nd pair.

Thanks,
Tom

 

[Swamp Thing]

Except, if you look closely, you may notice that pair #2 is a transmission line inverter.

I didn't look that closely myself . I didn't actually trace each wire from end to end, and I thought no one would.

I should have drawn color coded wired, or labeled them "A" and "B" for each end of each pair.