Antenna trap

[Driverj30t9]

Sorry it took me a little while to respond back on here. I do that sometimes.

I might add that antennas will "resonate" on both odd and even multiples of its fundamental frequency but the impedance will be more usable when fed with low impedance coax on the odd harmonics rather than the even. If your discussing an offset center fed or end fed with matching transformer you can generally use the antenna on most odd and even multiples with a usable low VSWR.

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Okay, thanks for clarifying that.

As mentioned, the radiation patterns from the various combinations gets busy and deserves its own thread.

I can imagine.

A half wave vertical *can* certainly be resonant, but as said, it is not an easy antenna to match, especially with coax cables.

Good for some that want a challenge. Or if perhaps there are benefits of a 180 degree antenna over quater wave. Higher radiation resistance maybe.

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The '5 Band Antenna' is a very popular concept. However, I think Driver, you are talking of wire dipoles here (though there are many multi- band verticals using similar designs.) Irregardless, you'll need to place 'traps' in them. I'll give you a "Coyote's" brief design description of one-- as if we were were building it together....

I can see why it would be popular, having all those bands at one's disposal to meddle with. Actually yes I was picturing dipoles when I was thinking about five banders. That would be interesting.
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Start with a half wave dipole approximating the physical length at 80 metres' (~3.6Mhz).... then place L/C 'traps' at the the half wave length of 40 (7.2Mhz.) Theses 'traps' will pass 80 metre's, blocking and resonating the shorter section to 40; On 20 thru 10, the antenna will be working on a capacitive reactance principle that I won't go into here (Heaven knows I done enuff damage).... - but it will work, and the SWR will be reasonable.......
.........................I'll let you explore that one.....

That's okay, I was going to dig back into that capacitive inductive reactance stuff again anyways. Perhaps another time. Your input was helpful and relevant enough.

Most of the time an antenna's SWR 'curve' is low for a few percent of the operating frequency-so the higher the frequency band the 'wider' in frequency(MHZ) terms. Many antenna designs (like multi band dipoles) used today have not changed for many years. The five band antenna likely is one from before hams got the 'WARC' bands in the early 80's. Physics don't change with time and while technology may change, the basic principles of electric and magnetic fields are the same as they always were.

That makes sense now. What are these WARC bands? They sound like these oddball bands, haven't really heard them mentioned much. What are they used for? That sounds kind of counter intuitive that they add bands but then use antennas designed for fewer bands.

Physics don't change with time and while technology may change, the basic principles of electric and magnetic fields are the same as they always were.

True statement.

Guess I am getting old-I remember the excitement of getting the 'new' bands and also the scary feeling knowing that a new 'rig' (a big chuck of cash back then) would be needed.

Or you just have more cash on hand now and when's the last time they added any new bands? There adding a 700mhz band? I don't know, not in the know. I know they added a band not long ago or there still going too. Phasing it in as we speak maybe?

I have delite'd for years in re-discovering all things historical.

History's interesting stuff. Thinking about all these realities that were happening in this world before you even existed. Other times, other places.

I think we too often sell ourselves short when we feel we must embrace the newest and latest.

Like relying on GPS and not learning how to use a map.

I've mention'd in these forums before how my great aunt was a Marconi Girl... the very fact she could communicate to ships at sea with rotary spark and coherer receivers absolutely astounds me... yet I really wish she could see how far her great niece has come.... for I am sure we'd have so much radio to talk about in common.

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It would be neat to contact a vessel from afar. Why stop at land? If not the ocean, maybe say Lake Superior. That dx stuff is something I know nothing about, maybe it sounds like it. My grandpa was a submarine morse code operator in WW2. Never met him, but I have the same interests.

"So that is what a Valve is !.... we've been hearing about Mr. Fleming....... ")

???? :/
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[wyShack]

the WARC bands added in the 80's were 30,17 and 12 meters. Hams also can use 5 'channels' in the 60 meter band. and just recently got access to 2200 meters (That's 137.5 -137.8 khz) and 630 meters (472-479 khz). The last two authorizations have power and antenna limits that are challenging. Few amateurs have antennas (or even radios) to operate on all the bands -or use an antenna that is 'tuned' rather than resonant.

Back then, there were many station on just one band (like 40 meters) and many transmitters were crystal controlled so you had quite a bit less flexibility-you would call CQ and then tune for response as the 'other end' may not have had a 'rock' to transmit where you did. That and tuning the tube finials in most radios added some complexity to getting on the air. I have fond memories but have to admit today is more convenient-may argue about better.

73

 

[majoco]

Having spent a few happy hours, even if a little frustrating, tuning up a nice new Hy-Gain 18AVT-WB 5 band vertical, I can tell you its not a trivial exercise. Back in the early 80's I didn't have access to an antenna analyser so it was done with the trusty VSWR meter. I thought I'd be smart and put a table and chair at the base of the antenna with a little Kenwood 120V and a car battery. Start with the 10m base section, a few trial positions and found the sweet spot, VSWR below 1.3, walked away to admire my walk and lo - the VSWR went up over 2! My body was detuning the 10m section as I was too close!

Anyway got it all sorted in the end, longer bit of coax and was good.

But to add to the confusion - you can't start with an 80m dipole, then just chop it and insert a 40m at the calculated spot. It may work fine on 40 but...80 won't be good. The inductance of the trap makes the 80m section too long...

 

[Driverj30t9]

the WARC bands added in the 80's were 30,17 and 12 meters. Hams also can use 5 'channels' in the 60 meter band. and just recently got access to 2200 meters (That's 137.5 -137.8 khz) and 630 meters (472-479 khz). The last two authorizations have power and antenna limits that are challenging. Few amateurs have antennas (or even radios) to operate on all the bands -or use an antenna that is 'tuned' rather than resonant.

II"ll have to give those WARC bands a try sometime, when I have my license of course.

QUOTE=wyShack;2957997] Back then, there were many station on just one band (like 40 meters) and many transmitters were crystal controlled so you had quite a bit less flexibility-you would call CQ and then tune for response as the 'other end' may not have had a 'rock' to transmit where you did. That and tuning the tube finials in most radios added some complexity to getting on the air. I have fond memories but have to admit today is more convenient-may argue.[/QUOTE]

Hmm, didn't know there was so much work involved. I'd say the radios now days are better because you don't have to fiddle with all that but it would be fun to use one of those radios from time to time.

Take care

Having spent a few happy hours, even if a little frustrating, tuning up a nice new Hy-Gain 18AVT-WB 5 band vertical, I can tell you its not a trivial exercise. Back in the early 80's I didn't have access to an antenna analyser so it was done with the trusty VSWR meter. I thought I'd be smart and put a table and chair at the base of the antenna with a little Kenwood 120V and a car battery. Start with the 10m base section, a few trial positions and found the sweet spot, VSWR below 1.3, walked away to admire my walk and lo - the VSWR went up over 2! My body was detuning the 10m section as I was too close!

Yeah tuning a 5 bander doesn't exactly sound like a stroll in the park. Funny story. Sounds like it's a good idea to always stay back from the antenna a ways when tuning then. It's nice when a broadcast station on the radio isn't coming in good though and being near the antenna improves reception. Good thing you caught it, I never heard of that being an issue until now. I guess any obstructions including ones self that won't normally be around the antenna are a bad thing when tuning. Wonder what a good distance away would be then?

Anyway got it all sorted in the end, longer bit of coax and was good.

Good good.

But to add to the confusion - you can't start with an 80m dipole, then just chop it and insert a 40m at the calculated spot. It may work fine on 40 but...80 won't be good. The inductance of the trap makes the 80m section too long...

Interesting. A bit of an skill antenna tuning.

 

[Lauri-CoyoteFrostbite]

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"But to add to the confusion - you can't start with an 80m dipole, then just chop it and insert a 40m at the calculated spot. It may work fine on 40 but...80 won't be good. The inductance of the trap makes the 80m section too long..."
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So true; I should have prefaced that general comment with more details. Unfortunately with my move here to the 'Swamp"--(Wash. DC) -- I did not bring any of my ham radio books with me- so instead I text'd a former intern, (she's a physicist and an Extra Class ham)----
This is her answer to my call for help.
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"You want a design for a 5 (ham) Band (no WARC's) dipole antenna?..... check your e-mails, Prof.... "
(parentheses mine)
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.........and this design was sent to me shortly.... I'll put it into these words.
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"Make a dipole, ~110 feet long, placing the inductor/capacitors (L/C's ) ~30 feet from the centre insulator. The coils (L) are 10 turns of #16 copper wire, ~2" in diameter, ~7 turns to the inch. (this approximates a value of 8 uH)"
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-------------(L/C)-----------------------------[ ]---------------------------(L/C)----------
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"The capacitors (C) are 50 to 70 pF and depending on the transmitter's power, consider using high voltage ceramics. This may be fed with coax cable- 72 Ohm preferr'd."
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"Be prepared to do some tuning !" .............was her final admonition.
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I make no guarantees about this antenna, but it comes from someone in whom I have implicit trust. But to the point of length, as can be seen, its considerably shorter than a standard 80 metre dipole.
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Oh, and a "Valve" , Driver ??--- that's a British term for a Tube..... they were first practically develop'd by John Fleming as a vacuum tube device for radio reception, ca: first years of the 20th Century.....
but you should also look up Lee de Forest and his tube designs
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Lauri
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[Lauri-CoyoteFrostbite]

I should have add'd something further, Driver, in my convoluted explanation of vertical antennas (sorry if I've created more confusion instead of clarity).... its about the comment on Half Wave Verticals (I personally like to think of them in terms of degrees...besides, if you say a "half wave"....oh so Pedestrian !... but say instead: "I'm experimenting with a 180 degree antenna"-- you become an instant Marconi !)
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As been aptly pointed out, a half wave ....opps... a 180 degree .... antenna is certainly resonate. But restricting this to the practical ground mount's vertical scenario; to match these beasts is a Herculean task. Yes, it certainly can be done, but the input impedance is so critcal, its so Bozo high; the bandwidth is so narrow, ---that any matching circuits (ie: coax to vertical) will have to be so-dead-on that if not, any variations will send that impedance match out to somewhere near Jupiter's orbit. Think rain/snow/wind, for start'rs...
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The the popular V/UHF 5/8's wave..... another 'Oppps'-- a 225 degree antenna- is in this league. But the much improved radiation pattern over a 90 degree makes it worth the matching effort. Plus the base impedance isn't nearly as bad for coax.
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Its been some time I've thought of this.,, so help me out Guys- aren't some of the Clear Channel Blowtorches on AM Radio using non-resonate antennas... 7/8th's (315 degree- or some other odd ball number) -- for their super low angles of radiation ??
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Given the right LC circuits you can match anything... but I'd steer clear of these goofy resonance challenges if you wish the simpler life...
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Lauri
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(I apologize if I'm a bit verbose-- an early evening's return from Georgetown- for those that know DC........ )
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[majoco]

Just like the popular 5/8th's 2m antenna - there's an inductance at the base - here's a video on how to make one - I didn't think she was serious at the start! Dodgy looking soldered joint onto the base though.

I tend to think of a quarter wave like this - consider the far end of the quarter wave - what can exist at an open circuit? Only voltage, no current - a high impedance. As you come back down the quarter wave, the voltage decreases, the current increases, the impedance is coming down as per ohm's law - when you get right to the bottom, in theory there's heaps of current but no voltage as it's shorted to ground. Move up a wee bit and you find 50ohms or whatever you're looking for. Now consider the half wave - the low impedance point is now in the centre - as you go down the half wave, the impedance is increasing again but in the opposite phase and as you get right down the bottom it's all voltage and no current again - very high impedance that is difficult to match. You can of course add another quarter wave as a matching stub - bend it round on itself and it's called a J-pole..... oh, no, not again....

Over simplified I know but it works for me!

 

[prcguy]

Within the HF bands it turns out there are known and easy ways to match end fed 1/2 wave or 180deg antennas. Its also convenient that the impedance of a half wave antenna repeats itself every 180 degrees, so a broad band matching circuit can work on multiple harmonics and give you more bands to operate with a good match to low impedance coax.

A half wave antenna in free space fed on one end has an impedance of a couple thousand ohms. Hams have been using broad band transformers with between a 50:1 and 64:1 ratio to match 50 ohm coax to around 2,500 to 3,200 ohms. A 100W version of the transformer cost about $2 to assemble and a 400W version about $10 and these work in the frequency range of about 5 to 30MHz with no tuning.

Make one of these transformers and you can simply attach a half wavelength of wire at any frequency within the transformers range and it will provide a good match to low impedance coax. About 16ft of wire hanging vertical will give you a 10M mono band antenna. About 33ft will give you a combo 10M and 20M antenna with no tuning or ground radials or grounding needed. About 64ft of wire will give you 40M, 20M, 10M and as a bonus it also matches well on 15M. You can also add traps to get bands that the single wire length would otherwise not provide.

On the fundamental or lowest band you cut the wire for the antenna will work great as a vertical with most of the radiation going out towards the horizon, but on higher harmonic bands you will get multiple gain lobes and nulls, making it not so good for local or DX. However, when you use this type of antenna horizontal it becomes much more useful on multiple bands and can take advantage of NVIS or high angle on 40m and low angle for DX on higher bands like 20 and 10M.

I can post links to instructions for building these magic transformers if your interested and its just another fascinating facet of the radio hobby and the joys of antenna's.

I should have add'd something further, Driver, in my convoluted explanation of vertical antennas (sorry if I've created more confusion instead of clarity).... its about the comment on Half Wave Verticals (I personally like to think of them in terms of degrees...besides, if you say a "half wave"....oh so Pedestrian !... but say instead: "I'm experimenting with a 180 degree antenna"-- you become an instant Marconi !)
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As been aptly pointed out, a half wave ....opps... a 180 degree .... antenna is certainly resonate. But restricting this to the practical ground mount's vertical scenario; to match these beasts is a Herculean task. Yes, it certainly can be done, but the input impedance is so critcal, its so Bozo high; the bandwidth is so narrow, ---that any matching circuits (ie: coax to vertical) will have to be so-dead-on that if not, any variations will send that impedance match out to somewhere near Jupiter's orbit. Think rain/snow/wind, for start'rs...
.
The the popular V/UHF 5/8's wave..... another 'Oppps'-- a 225 degree antenna- is in this league. But the much improved radiation pattern over a 90 degree makes it worth the matching effort. Plus the base impedance isn't nearly as bad for coax.
.
Its been some time I've thought of this.,, so help me out Guys- aren't some of the Clear Channel Blowtorches on AM Radio using non-resonate antennas... 7/8th's (315 degree- or some other odd ball number) -- for their super low angles of radiation ??
.
Given the right LC circuits you can match anything... but I'd steer clear of these goofy resonance challenges if you wish the simpler life...
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Lauri
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(I apologize if I'm a bit verbose-- an early evening's return from Georgetown- for those that know DC........ )
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[Driverj30t9]

I"Make a dipole, ~110 feet long, placing the inductor/capacitors (L/C's ) ~30 feet from the centre insulator. The coils (L) are 10 turns of #16 copper wire, ~2" in diameter, ~7 turns to the inch. (this approximates a value of 8 uH)"

-------------(L/C)-----------------------------[ ]---------------------------(L/C)----------

I would maybe try that design out if I had 110ft of space. Some day perhaps.

.Oh, and a "Valve" , Driver ??--- that's a British term for a Tube..... they were first practically develop'd by John Fleming as a vacuum tube device for radio reception, ca: first years of the 20th Century..... but you should also look up Lee de Forest and his tube designs

Oh right. I"ll read up on them. I find how people figured all this stuff out almost as interesting as the science itself.

I should have add'd something further, Driver, in my convoluted explanation of vertical antennas (sorry if I've created more confusion instead of clarity).... its about the comment on Half Wave Verticals (I personally like to think of them in terms of degrees...besides, if you say a "half wave"....oh so Pedestrian !... but say instead: "I'm experimenting with a 180 degree antenna"-- you become an instant Marconi !)

Lol, or at least appear to be one, inch a little a closer to being one. I like saying half wave, 1/2 wave, 180 degree. Just mix it up, keeps things interesting, same with the other wavelengths. Don't engineers use radians? Or no?

As been aptly pointed out, a half wave ....opps... a 180 degree .... antenna is certainly resonate. But restricting this to the practical ground mount's vertical scenario; to match these beasts is a Herculean task. Yes, it certainly can be done, but the input impedance is so critcal, its so Bozo high; the bandwidth is so narrow, ---that any matching circuits (ie: coax to vertical) will have to be so-dead-on that if not, any variations will send that impedance match out to somewhere near Jupiter's orbit. Think rain/snow/wind, for start'rs...

How come the bandwidth would be narrow?
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The the popular V/UHF 5/8's wave..... another 'Oppps'-- a 225 degree antenna- is in this league. But the much improved radiation pattern over a 90 degree makes it worth the matching effort. Plus the base impedance isn't nearly as bad for coax.

If you had the exact same type of antennas, placed exactly in the same spot, just swapped them out to test them, a 1/4, 1/2, 3/4, or 5/8, which would give the best radiation pattern? Might as well ask since I'm here and it's fresh in my mind.

Given the right LC circuits you can match anything... but I'd steer clear of these goofy resonance challenges if you wish the simpler life...

Will see, probably take the easier route, but I would also find enjoyment in learning how to match up stuff.

Just like the popular 5/8th's 2m antenna - there's an inductance at the base - here's a video on how to make one - I didn't think she was serious at the start! Dodgy looking soldered joint onto the base though.

I"ll give those 5/8th vids a look.

I tend to think of a quarter wave like this - consider the far end of the quarter wave - what can exist at an open circuit? Only voltage, no current - a high impedance. As you come back down the quarter wave, the voltage decreases, the current increases, the impedance is coming down as per ohm's law - when you get right to the bottom, in theory there's heaps of current but no voltage as it's shorted to ground. Move up a wee bit and you find 50ohms or whatever you're looking for. Now consider the half wave - the low impedance point is now in the centre - as you go down the half wave, the impedance is increasing again but in the opposite phase and as you get right down the bottom it's all voltage and no current again - very high impedance that is difficult to match. You can of course add another quarter wave as a matching stub - bend it round on itself and it's called a J-pole..... oh, no, not again....

QUOTE=majoco;2959134]Over simplified I know but it works for me![/QUOTE]

That's a good way to think of it, it made me think more of the impedance along an antenna related to the current and voltage.

QUOTE=majoco;2959134]You can of course add another quarter wave as a matching stub - bend it round on itself and it's called a J-pole..... oh, no, not again.... [/QUOTE]

Yeah, stop right there sir. That"ll do for now.

Within the HF bands it turns out there are known and easy ways to match end fed 1/2 wave or 180deg antennas.

Excellent.

Its also convenient that the impedance of a half wave antenna repeats itself every 180 degrees, so a broad band matching circuit can work on multiple harmonics and give you more bands to operate with a good match to low impedance coax.

Why's it convenient that the impedance repeats every 180 degrees? Don't know if I'm following you just yet. I would only need one transformer at the base of the antenna if I'm also using harmonic frequencies besides just the fundamental?

A half wave antenna in free space fed on one end has an impedance of a couple thousand ohms. Hams have been using broad band transformers with between a 50:1 and 64:1 ratio to match 50 ohm coax to around 2,500 to 3,200 ohms. A 100W version of the transformer cost about $2 to assemble and a 400W version about $10 and these work in the frequency range of about 5 to 30MHz with no tuning.

Brilliant.

Make one of these transformers and you can simply attach a half wavelength of wire at any frequency within the transformers range and it will provide a good match to low impedance coax. About 16ft of wire hanging vertical will give you a 10M mono band antenna. About 33ft will give you a combo 10M and 20M antenna with no tuning or ground radials or grounding needed. About 64ft of wire will give you 40M, 20M, 10M and as a bonus it also matches well on 15M. You can also add traps to get bands that the single wire length would otherwise not provide.

A five bander. I will refer back to this if the time arises.

On the fundamental or lowest band you cut the wire for the antenna will work great as a vertical with most of the radiation going out towards the horizon, but on higher harmonic bands you will get multiple gain lobes and nulls, making it not so good for local or DX. However, when you use this type of antenna horizontal it becomes much more useful on multiple bands and can take advantage of NVIS or high angle on 40m and low angle for DX on higher bands like 20 and 10M.

How come the multiple nulls and lobes on higher harmonics?

I can post links to instructions for building these magic transformers if your interested and its just another fascinating facet of the radio hobby and the joys of antenna's.

Sure I"ll get the links from you. I was planning on messing around with this stuff anyways, bought a book on it that I haven't hardly picked up to read yet. Don't know if it talks about the ratio transformers your referring to above though.

 

[majoco]

How come the multiple nulls and lobes on higher harmonics?

Aaaah - not easily explained, but here goes.... Consider a horizontal half-wave dipole, low impedance in the centre so maximum current there, the current produces the electromagnetic field which radiates away at right angles to the centre of antenna but nothing from the ends as the current is low, so you can think of it as a 'figure of eight' with the antenna at the centre from a plan view - in 3 dimensions it's like a donut with a very small hole in the middle. Now if you take the third harmonic, there are now three current maximums but of opposite phases, say + in the middle and - at the two ends. Now we still have the 'figure of 8' from the centre current maximum but now there are two more from the ends BUT as they are of opposite phase they tend to cancel at the "joins" so you get a null and the two new lobes tend to angle away from the centre lobe which also gets thinner and longer giving you more signal at a distance.
The electrostatic wave from the voltage nodes also produce a similar pattern which tends to propagate better but we're not going there today thank goodness!

 

[Driverj30t9]

Oofta, yeah gonna have to look at some diagrams on that to get a better undering and read up about it.

 

[Driverj30t9]

I see what your saying though. Less radiation from the ends, more from the center. Figure 8.