# Analysis of km long cable antenna?

1. Sep 25, 2015

### houlahound

I can get some old massive size power transmission cable straight out of the power station. I have the space to erect some posts and go just over 500m but with permission I might get 1km or more.

Will use a metal pipe in the ground for ground.

This is not technically a dipole, would a dipole be better.

What formal problem is this configuration and how would I simulate its performance and optimise it.

Does the large diameter of the aluminium cable make a better antenna than just regular copper wire for electronic circuits?

The cable will need serious infrastructure to hold it up.

I want to simulate if a V antenna a dipole or my proposed single straight antenna is best.

2. Sep 25, 2015

### Staff: Mentor

You know that taking power from the power company and not paying for it is illegal, right?

3. Sep 25, 2015

### houlahound

This is radio frequency so far from power transmission lines that power is zero from power station. I meant I can acquire scrap cable that was used for power transmission.

Have cable have space of course I am going to build the mother of all antennas.

Want to tune to very low frequencies

4. Sep 25, 2015

### davenn

you understand for a dipole of that length, the resonant frequency is going to be very low ?

a 500m half wave dipole (250m per side) has a resonant freq of 300 kHz
a 1000m half wave dipole (500m per side) has a resonant freq of 150 kHz

what are you planning to listen to down there ?

Dave

5. Sep 25, 2015

### Staff: Mentor

Submarine traffic. Well, except for the part where all that traffic is encrypted with a billion bit encryption...

6. Sep 26, 2015

### houlahound

an encoded signal at those frequency would be cool to detect.

submarines did cross my mind. a HAM apparently listens to earthquakes but I am yet to talk to him personally.

there is apparently quite a bit of natural terrestrial and atmospheric activity.

anything would be super cool, I like scanning for noise and anomalous signals. I don't really care to listen to human traffic.

7. Sep 26, 2015

### Baluncore

If you can legally get the used power cable for free, then take it and sell it for scrap. The cost of physically supporting it is just to great.

For listening, you are better to build an antenna from thinner wire. Transmit antennas are usually large diameter bare metal to reduce corona fires, receive antennas are often insulated to reduce electrical noise from dust and rain. Depending on where on Earth you are, there will be other climatic constraints on antenna design.

Most encrypted signals are now on microwave satellite links or on optic fibre links where there is plenty of internet traffic to hide them. Listening to encrypted signals is now pointless. Even if you could crack the code, you could not use the information without being classified as a terrorist.

What do you consider to be Very Low Frequencies? You need to specify the frequency range you are interested in. There is quite a bit of activity below 500kHz. Once the frequency and polarisation are specified we can suggest optimum antennas for listening to those signals.

You might see how many aeronautical NDBs you can hear between 190 kHz and 535 kHz. Then see if you can do direction finding on them.
https://en.wikipedia.org/wiki/Non-directional_beacon

Or maybe go to the LF amateur allocation and get your antennas and receive equipment working on 136 kHz.
https://en.wikipedia.org/wiki/2200-meter_band

Your biggest problem below 1 MHz will be getting rid of the noise from the ubiquitous switching power supplies and and high efficiency light globes.

8. Sep 26, 2015

### houlahound

9. Sep 26, 2015

### nsaspook

We normally used loop antennas instead of a long wire for LF broadcast reception on the ship. Using frequency-division multiplex we could compress many (16 channels) TTY channels into one voice channel using a UCC-1.
http://www.virhistory.com/navy/rtty-mux-ucc1.htm
The sound of one (called the tone-pack long before the music guys used the term) is very distinct if you know what to listen for. Normally we also ran frequency duplex diversity with two inputs to the Mux receiver as it had a separate time delay adjustment for the input channels so the signals from two distant stations could be synchronized. This required great stability in the complete timing chain with receivers and transmitters phased locked to a few HZ so the decryption gear would maintain lock on the stream encryption as signals faded from one transmitter to the next.

Sub antennas:
http://www.sippican.com/contentmgr/showdetails.php/id/314

10. Sep 27, 2015

### sophiecentaur

If you have a lot of cable and a lot of space to carry it (many hundreds of metres) then, for very long wavelengths, it may be worth while considering a Beverage Antenna. This consists of a long wire, suspended on a series of poles (or even laid on the ground, aamof). It is a travelling wave antenna and it would not need a complicated tuning network. All you need to do is terminate the far end to ground with a fairly good match and put your receiver at the other end with good earth connection and you will have a directional receiving antenna with a main beam along the direction of the wire. Although used for lf and mf, the theory would also apply to vlf. You could connect a receiver at both ends an look both ways at once.
I think that bulky power cable may be a bit over the top for a receiving antenna and the ground losses on a long wire near the ground would probably dominate.

11. Sep 27, 2015

### davenn

and also just good ol' 50/60 Hz mains hum

All the atmospherics receiving I did in years gone by was with a multi turn loop antenna
A 50m length of 50 pair ( 100 wire) telephone cable connect each wire in series to produce a 100 turn loop antenna
drive out to a quiet site well away from towns and cities, power lines etc, lay the loop out on the ground and hook up the receiver
Receiving atmospherics ... whistlers, dawn chorus etc around the 20kHz freq was no problem at all

Dave

12. Sep 27, 2015

### Baluncore

Beverage antennas are highly dependent on ground conditions so without a ground plane their performance varies seasonally. They are well worth investigating but are outperformed by tapered rhombic antennas. That is because the virtual ground reflection cancels almost all their horizontal radiation. Also the dipole radiation pattern has a deep null in the direction of the wire at lower frequencies such as LF. Beverage antennas do work very well at MW and SW when they are two or more wavelengths long and have a ground wire on the surface. You do need an awfully long wire to Beverage at LF, and they do pick up a lot of local man-made electrical noise.

For vertically polarised VLF I use a big radial capacity hat on a vertical mast above a surface ground plane. The capacity-hat and ground-plane make a capacitor that can be brought to resonance by an adjustable inductor in the mast. For vertical signals it outperforms everything else I have tried on VLF. It can be tuned to any frequency by introducing sufficient inductance. That is an application for a variometer, it maybe servo-driven from the receiver position. https://en.wikipedia.org/wiki/Transformer_types#Variometer_and_variocoupler
NBD antennas often apply the capacity hat technique because it is non-directional. Be aware that highly tuned transmit antennas can easily generate corona discharge and start fires if not carefully engineered.

For horizontal radiation LF antennas I prefer a large triangular multi-turn loop. The top is supported by a mast, the bottom corners at short posts. The inductive loop is tuned with a variable capacitor at the base. The number of turns selected can be switched depending on the inductance needed for the frequency being received. One turn is used as the balanced feed to the receiver. To eliminate local noise the entire loop can be inside a metal screen that is earthed symmetrically at the bottom of the tower, but there must be a break in the screen at the top to prevent a shorted turn. The loop antenna may need to swing through an azimuth of 180° to null an interfering signal. It can be walked around to different bottom posts.
Two of those loops on the same tower, one in the N–S plane, the other in the E–W, make a really good crossed–loop, (Bellini–Tosi), direction finder for LF and VLF. https://en.wikipedia.org/wiki/Bellini–Tosi_direction_finder#Bellini.E2.80.93Tosi

13. Sep 27, 2015

### Baluncore

Schumann Resonances are natural Earth–Ionosphere resonances that are plucked by lightning strikes. They range from 7Hz to 40Hz. For example spectra, google images 'Schumann Resonances'

You can listen to them below the 50Hz and 60Hz power frequencies with an A to D converter such as a modified sound card. You will need a notch filter to remove your local power frequency interference, but with digital signal processing, such as power spectrum accumulation you can now dig deep under the noise.

14. Oct 6, 2015

### houlahound

I stopped getting notifications and didn't realise there were new replies. Will take weeks to read them and links.

Thank you all.

15. Oct 7, 2015

At 300 hz - that would take like 900 hours to send a message ...

16. Oct 7, 2015

### sophiecentaur

@Baluncore. I don't hold a particular torch for Beverages but, given the circumstances (a vast area for the antenna), it could be attractive. You could set off in a truck, full of 2m or 3m poles and a reel of cable and hammer in the poles at intervals. Put a simple resistive termination to an earth spike by the last pole and work back to the start, hanging the wire as you go. It's a pretty unfussy design - wide band and match tolerant. Some early versions worked with no poles, being laid over scrub. Low wires are more lossy and there is a limit to effectiveness of very long wires. For someone who's not familiar with aerial (I.e. Up in the air) construction and measurement, it would be s significant step up from the proverbial bed frame. It is restricted to VP, I think. It can be vulnerable to cattle pushing the poles over, whilst scratching themselves, I've heard.
Worth a punt, I'd say.

17. Oct 7, 2015

### nsaspook

I know it's a gag but that's just about how long the streams pseudorandom sequence that's used to encode the short message is from one 'key' card that's changed daily for extra security.

18. Oct 7, 2015

### Baluncore

On HF or for casual listening, yes. But from years of experience at long wavelengths, a loop on the ground that can be tuned works much better than a travelling wave antenna that only gets one very poor dBi chance to radiate the energy. I made my original loop from 1 km of 50 mm2 used power cable. When the copper price peaked I pocketed the investment, and replaced it with a resonant vertical. A vertical LW antenna needs only two trees 50 to 100 metres apart. One wire is thrown over the trees and a separate wire is laid on the ground. A single “T stem” wire drops from the middle of the top wire to the inductive tuner/coupler at the middle of the bottom wire. It is a simple vertical capacitor tuned by the stem inductance, easy to install, efficient and cheap.

19. Oct 7, 2015

### Baluncore

With billion bit encryption, you can encode one 5 letter group by itself.
With a 300 Hz bandwidth it would take less than one tenth of a second to transmit that encrypted group.

20. Oct 7, 2015

I was thinking the block and the key wold both need to be a billion bits - but just the key?

21. Oct 7, 2015

### houlahound

Curious, what equation related bandwidth to frequency. High bandwidth is always at high frequencies eg television and I get you need more bandwidth for video than audio but whatvis the basic equation.

22. Oct 7, 2015

### sophiecentaur

The way the RF spectrum is shared out between users is based on practical considerations (and a lot of politics). The bandwidth used at a particular transmitting frequency is often based on what the technology permits. It is broadly a matter of 'fractional bandwidth'. That forces the use of high frequency carriers for wide bandwidth signals. Optical frequencies allow very wide band signalling but a 1MHz carrier is limited to only a few kHz of bandwidth.

23. Oct 7, 2015

### Baluncore

If you have a predetermined key schedule, then you do not need to transmit more than one encrypted word.

The bandwidth of a channel determines it's capacity, it is approximately the bit-rate.
The reciprocal of the BW is the settling time for a symbol.
Under good conditions you can squeeze maybe 6 bits into one symbol by using QAM.

24. Oct 7, 2015