DIrectional Antennas

The directional antenna idea comes up occasionally on various forums.

Commercial AM broadcast stations achieve a directional pattern by using multiple towers.The towers are usually built in a straight line. Then the transmitter signal is distributed to the towers over separate coax lines. The trick is that the coax lines are not all the same length. Coax delays a signal directly proportionally to its length. By precisely adjusting the length of each separate coax run, the phase of the signal can be controlled at each of the antennas in the array. the result is that the combined radiated signal from all antennas is cancelled in some directions and reinforced in other directions producing a directional pattern.

Part 15 antenna phasing could theoretically be done the same way, but not leagally by the 3 meter rule, and determination of the correct length of each coax run would be a very difficult problem to solve.

Another idea I have seen is to set up multiple transmitter/antennas in a similar array and spacing them precisely to achieve the required phase differences. The killer for this idea is that the phase of all tranmitters must be PRECICELY controlled. This is not achievable by relatively simple circuits. You can phase-lock all the transmitters to a single frequency source over equal length lines, but relatively simple circuits will not lock exactly due to circuit variations and low-level noise on the common frequency distribution line. The result is some amount of "jitter" of the frequency of each transmitter. Phased-array antennas won't work with jitter. Coax distribution in commercial transmitters doesn't have jitter.

Commercial AM broadcast stations achieve a directional pattern by using multiple towers.The towers are usually built in a straight line. Then the transmitter signal is distributed to the towers over separate coax lines. The trick is that the coax lines are not all the same length. Coax delays a signal directly proportionally to its length. By precisely adjusting the length of each separate coax run, the phase of the signal can be controlled at each of the antennas in the array. the result is that the combined radiated signal from all antennas is cancelled in some directions and reinforced in other directions producing a directional pattern. (etc)

To clarify, the electrical length of the coax run feeding each tower of a directional AM broadcast array can be whatever length is necessary for site layout purposes. Those coax lengths are important to developing the directional pattern, but the phasing of the towers, and the currents in each tower are controlled by adjustments to variable Ls and Cs in a large "phasor" cabinet in the transmitter room, which is driven by the transmitter. Each tower also has a network to match the base Z of the tower to the Z of the coax feedline.
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Hi Everyone,

Using a specific length of coax is still the easiest, most practical way of adjusting the phase of a signal at a tower for directional purposes, especially for experimenters. The key to success is knowing the exact velocity factor of the coax as well as using the standard formulas for wavelength. The problem is that it simply doesn't fit into Part 15 rules.

I thought I had a wonderful bit of insight after reading PhilB's remarks about using a common oscillator circuit. I was thinking of adjusting the length of the common oscillator circuit leads to each transmitter as a phase adjustment. But I suppose the length of the leads might have too big of an effect on the exact frequency of the more distant transmitter or the level might be so low that the second transmitter might not even operate.

Using a specific length of coax is still the easiest, most practical way of adjusting the phase of a signal at a tower for directional purposes, especially for experimenters. The key to success is knowing the exact velocity factor of the coax as well as using the standard formulas for wavelength. The problem is that it simply doesn't fit into Part 15 rules. etc

And that isn't the only problem with using just coax lengths to set up directional arrays, because there is more to this than setting the phase at each radiator. The number and physical separation of the radiators, their geometric arrangement with respect to true north, the currents and the phases in each radiator all react together to produce the end result. The radiators all interact with each other due to mutual coupling between them. Knowing where to start even to design one takes some complex mathematics, and getting them set up correctly takes good test equipment. A lot of experience in this is almost essential to anyone considering it -- not to mention some deep pockets.
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SO what your saying is there is no single directional antenna, like ther eis on 2.4 ghz..

ANd even if there were this type of antenna it would be very dificult to phase them correctly.

SO how does the rangemaster sync the signals for a wide area coverage.. I would think some one would build this feature i to a LPAM TX for us DIY folks, I for one would love to be able to afford a couple TX and place them over a wide area, but at 1k per rangemaster I have a serious cash issue there..

I posted a thread caller "rangemaster clone", and it got no responce, has anyone tried reverse engineering ti to see how they do this feat.

I would think if some one made this as a kit or a prebuilt for under 300 bucks there would be a massive insurgance of LPAM broadcasters wishing to expand there range..

If they were that inexpencsive i would be very willing to place them on listeners property that would be willing to donate the AC power and pole in there yard.. IM sure many listeners would be very willing to do this for there station as they would have a primo signal in there backyard.. what a bonus for a very small amout of ac power..

Jason

SO how does the rangemaster sync the signals for a wide area coverage.

If two Part 15 AM systems broadcast on the same nominal frequency, and their coverage areas overlap, receivers in the overlap area will have a heterodyne (tone) in the program audio that is the difference frequency in hertz between the exact carrier frequencies of the two transmitters.

Program modulation also has to be phased correctly on each tx in the system. Otherwise the sidebands of the different transmitters in the system can tend to cancel each other, and create a lot of distortion in receivers.

Phase-locking the carriers onto the same frequency will eliminate the heterodyne, but will not affect another issue with this kind of system. Radiation from the nearby antennas will add in some directions, and cancel in other directions. This happens even when the txs are phase-locked, and is related to the lengths of the propagation paths in wavelengths from each receiver location back to each tx. If the signals arrive in phase, they add. If they are out of phase, they cancel.

This is a totally different issue from the heterodyne described above, and can result in fairly large geographic sectors with poor, or no signal. The Rangemaster approach does not appear to recognize this, at least from the info on their website (Keith Hamilton - pls correct me if I'm wrong).

Truly synchronizing two txs has been done in commercial radio (example WBZ, Boston), but elaborate means were used to be sure that the interference zones generated by the two phase-locked carriers were directed away from areas with high population. And trying to do that with more than two nearby txs would be very problematic.

Part 15-ers might get better coverage results by installing a very good ground system for a single radiator than by installing "sync-ed" txs. This refers to the resistance of the connection of whatever constitutes the ground system (radials, etc) to physical earth -- not just using a larger gauge wire to connect to a cold water pipe, or whatever. Removing 10 or 20 ohms of resistance in the ground system can give a large improvement in radiation efficiency, due to the 1/10 of an ohm or so of radiation resistance for most Part 15 radiators.
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Hi, Rich..

I'm curious.. Say if I had (4) Part-15 xmtrs that were close togather over the same ground system, all of the reference osc. were from a common source. (all in sync) Would they enhance each other?

You might try burying more ground radials in the direction towards your target area. In my CB days, a rear mounted car antenna always gave better reception towards the front.

Say if I had (4) Part-15 xmtrs that were close togather over the same ground system, all of the reference osc. were from a common source. (all in sync) Would they enhance each other?

Yes, because this is the electrical equivalent of applying 4X the legal Part 15 power to a large cross-section structure acting as a single radiator. So the system would not be Part 15 compliant.

Increasing radiated power by 4X only doubles field strength (other conditions equal). Probably you could get more of an improvement than that just by installing a very good radial ground system to use with a single Part 15 tx and radiator.
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OK so what would be considered the perfect part 15 antenna and ground

I already have 8 - 10 foot radials and have plans for a sstran style antenna...

IF money was no object for a part i5 compliant what would be ideal

I mean should i burry chicken wire over my entire yard and ground rods in a 10 foot grid accross my entire yard.. I may take months to do but I could cover a acer with chicken wire... THat would be the best i could do on my property..

I coule elivat my TX and antenna up 20 feet or so..

As for the antenna if money were no object what would be the best options..

And also whats the max range in theroy if all conditions were perfect.

I guess with in the limits how can we make our stations the best possable... then with in those guide lines we would have to make the call of what were willing to do and can afford.

Jason

If money were no object... (and time and other resources...)

I am not an expert here, but I believe that commercial AM broadcasters use lots and lots of ground radials, like 360 radials (one for each compass degree).

And from what I gather on these boards, ground radials don't count against the 3 meter antenna length limitation and you can't have too much grounding.

How about mixing metal filings into your soil on your property.. Increasing the conductivity.. or watering your grounding system..

Ok im getting stupid now but hey its fun to contimplate 🙂

Does the grounding system actually have to be burried undreground... I have a manufatured home 80' x 28' and the skirting we put on it is all aluminum and all screwed together and staked into the ground every 4 feet or so going about 6 inches into the ground... so should I run a few grounding radials to it and conect it up to the ground system..

How about hooking to the telephone systems ground?? that would give me a huge ground system, all copper and in every home in the town LOL 🙂

We have plastic and concrete sewer systems here thats a bummer.. but I have my own well, all steel case going down many many many feet. What about that..

What about if you have a steel shed or roof

good or bad ideas, what items that we already have would make great grounds besides digging up the entire yard..

Would any of these ideas help us out..

Jason

Hi Jason:

I think you'll have good luck simply pounding 8 foot copper grounding rods into the ground near your antenna mast. They cost about $13 apiece around here. It's quick and easy. With a post hammer, you can quite literally pound one into the ground in about 5 minutes or less. Just wear some earplugs.

I've only done two and it seems to have helped. I'll probably do two more this weekend.

I used U-bolts to fasten the copper grounding wire to the galvanized mast pipe. I would have preferred the equivalent of pipe grounding clamps like I used on the 1/2" grounding rods, but couldn't find anything like that for the 1 1/2" mast pipe.

Regards,

Scott

I guess for the time being im going to test some of thise above ideas.. like hooking up to my skirting... telephone systems and maybe some other creative ideas... If i come up with a winner i will post it..

As for the future, yes I will improve my grounding system probably next year, i will redo the entire antenna and ground system..

I think my first step is to make a SStran antenna, to there recomendations then I can tackle the rest of the grounding system..

Jason