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Author Topic: linear Loaded Dipole  (Read 23234 times)
WA9YSD
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« on: January 10, 2012, 09:04:45 AM »

Now any one know what the magic lengths will be to make an antenna work like this one on 160M and 80M ?

I converted my 160M VFTD over to a Linear Loaded Dipole by taking out the terminator and replacing the balun and adding 40 feet of home made ladder line.

The Cobra Ultra light has issues on both 160M and 80M cause high impedances and high loss.

Jim K9TF
« Last Edit: January 22, 2012, 11:29:32 AM by WA9YSD » Logged
KB4QAA
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« Reply #1 on: January 10, 2012, 12:42:13 PM »

Why do you want to use a linear loaded dipole?   Is space a limiting factor?

Linear loading an antenna does result in lower efficiency compared to a normal dipole.

Feedpoint impedance is around 300 ohm.

Have you purchased the ARRL Antenna Handbook and ON4UN's Low Band Dx'ing?  These should be starting points for anyone experimenting with low band antennas.  They have the designs and theory for dozens of proven antennas.

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KB4QAA
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« Reply #2 on: January 10, 2012, 01:01:28 PM »

http://www.w8ji.com/radiation_resistance.htm
Tom's comments on folded dipoles and linear loading

The ARRL Handbook discusses folded dipoles but not linear loading.  (take that for what is worth).   Folded dipole formula is the same as regular dipole.

ON4UN dedicates two paragraphs to linear loaded dipoles.  He notes that they are lower efficiency and refers to Tom W8JI's comments that they are a poor loading coil format.  No formula is provided.  ON4UN says an NEC4 program is needed to avoid significant design errors.

The conventional alternative to a shorter antenna capable of operating on 160m and 80m would be a regular dipole with 160m loading coils and 80m traps.  These can be made or purchased.

73, bill
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WB6BYU
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« Reply #3 on: January 10, 2012, 01:45:49 PM »

But this really isn't "linear loading".  The antenna is close to 1/2 wavelength overall, in the
form of an open folded dipole.  That's quite different than using an inductive stub to
bring a shortened antenna to resonance.

On 160m each half of the antenna runs out about 1/4 wavelength then folds back on
itself.  The currents in the two wires nearly cancel each other, giving a low input impedance
(in spite of the fact that the total wire in each half is close to 1/2 wavelength) and
poor efficiency.

On 80m each side of the antenna is about 1/2 wavelength long.  The folded back wire
carries nearly the same current distribution as the driven wire, so the pattern and impedance
are very close to two half waves in phase.


In both cases the antenna would work at least as well, if not better, using a single wire,
or shorting both wires together on each side of the feedpoint.
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KE2TR
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« Reply #4 on: January 10, 2012, 09:24:20 PM »

I have to laugh here about the coments on linear loading, for many years HyGain,KLM,M2&F12 have been building 40mtr beams using linear loaded elements and KLM/M2&F12 have built some 75/80 beams with the same type of laoding with very good results. Sure is you read W8JI's coments on using coils you can get some excellent results but READ what he say, those coils have to be of very HiQ, which on the lowbands takes some doing with conductor size and size of the coil as well with about as perfect of a form as you can get. I have used both coils and linear loading on a pair of 75mtr verticals that were about 42 ft high, to me with results I had with both center loading with coils and LL at the center of the element to be honest LL provided better bandwidth, easier to match and performed much better that coil loading. Hey coils were the easy of the two and just because the ARRL doesn't put much info about LL in there handbook doesnt say didly, they make that book for the masses, its garden variety and I think the reason most dont try LL is its harder to model on most computer software so you must go back to the old fassion way of trial and error. Just rememeber if you use base loading with coils at the hi current point of the antenna that will severly have poorer performance over center loading either coils or LL. BTW ON4UN's book put more than just a few lines about LL, maybe not so much on dipoles but he also covers vertical LL as well. The problem is most hams want everything done for them, many of these books give great info but then its up to you to apply that info and try new ideas and sometimes these ideas are not always found on computer modeling, LL can be used with good results just do it at the center of each half of the dipole if you do.
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VE7RF
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« Reply #5 on: January 11, 2012, 01:20:28 AM »

I have to laugh here about the comment's on linear loading, for many years HyGain,KLM,M2&F12 have been building 40mtr beams using linear loaded elements and KLM/M2&F12 have built some 75/80 beams with the same type of laoding with very good results. Sure is you read W8JI's coments on using coils you can get some excellent results but READ what he say, those coils have to be of very HiQ, which on the lowbands takes some doing with conductor size and size of the coil as well with about as perfect of a form as you can get. I have used both coils and linear loading on a pair of 75mtr verticals that were about 42 ft high, to me with results I had with both center loading with coils and LL at the center of the element to be honest LL provided better bandwidth, easier to match and performed much better that coil loading. Hey coils were the easy of the two and just because the ARRL doesn't put much info about LL in there handbook doesn't say diddly, they make that book for the masses, its garden variety and I think the reason most don't try LL is its harder to model on most computer software so you must go back to the old fashioned way of trial and error. Just remember if you use base loading with coils at the hi current point of the antenna that will severely have poorer performance over center loading either coils or LL. BTW ON4UN's book put more than just a few lines about LL, maybe not so much on dipoles but he also covers vertical LL as well. The problem is most hams want everything done for them, many of these books give great info but then its up to you to apply that info and try new ideas and sometimes these ideas are not always found on computer modeling, LL can be used with good results just do it at the center of each half of the dipole if you do.

## agreed.  I wouldn't put much, if any stock in an arrl ant book.  Look at M2's latest detailed notes on LL with their 80m yagi's.  They USED to use 10 ga copper wire for LL.  Now they use 3/8" AL tubing for the LL. [which is also the truss line support] . M2 found that by using 3/8" al tubing for the LL, that gain went up .7db !   They also found out that where the 3/8" tubing works best is in the parasitic eles..like the ref + dir.  On the DE, it didn't buy you any improvement. They  use it on the DE anyway, since you require something for the over head truss.  They also claim there is no improvement by using center loaded HIGH-Q coils..like > 600, made from 1/4" cu tubing...VS LL made from 3/8" tubing.  Even if center loaded high Q coils are used, you STILL require a truss support on em.  SO LL DOES work, you just have to use bigger diam material than 10 ga wire. 

##  having said all that.  I rebuilt my F-12 EF-180B 68' long 80m rotary dipole.  I tossed the LL wires completely, and replaced em with 16' long T bars [ capacity hats].  The balance of the loading is done via a pair of  motor driven compressible coils, made by seco systems. The pair of coils is located right at the feedpoint. They are  made from plastic coated 1/4" copper tubing coils in an enclosure. Each coil is adjustable from 6-12 uh.  SWR is flat from 3200 khz to 4100 khz.  An Ameritron SDC-102 screwdriver controller, operates the motor driven assy.  It has a 4 digit digital turns counter display + 10 x pre-sets.  And no, you can't blow it up with 15 kw either.

later... Jim  VE7RF
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W8JI
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« Reply #6 on: January 11, 2012, 08:03:09 AM »

I have to laugh here about the coments on linear loading, for many years HyGain,KLM,M2&F12 have been building 40mtr beams using linear loaded elements and KLM/M2&F12 have built some 75/80 beams with the same type of laoding with very good results.


"Good results" is a meaningless term. Anything that makes contacts and has low SWR can be called "good results".

Quote
you read W8JI's coments on using coils you can get some excellent results but READ what he say, those coils have to be of very HiQ, which on the lowbands takes some doing with conductor size and size of the coil as well with about as perfect of a form as you can get.


The same size wire in linear loading, at the same effective location in the element, will always be less efficient than a coil of the same wire.

This is because the linear loading really is just an inductor all stretched out of shape. We lose the advanatage of magnetic  coupling increasing inductance when we make it linear, and that requires more wire for the same inductance. It also adds more shunt capacitance, and that increases circulating currents.

There is no electrical advantage to linear loading over the same wire in a coil. Q is not even close.

Quote
I have used both coils and linear loading on a pair of 75mtr verticals that were about 42 ft high, to me with results I had with both center loading with coils and LL at the center of the element to be honest LL provided better bandwidth, easier to match and performed much better that coil loading.


I wouldn't even guess why the antenna had better signal level performance, but the bandwidth increase would come from additional losses.

Linear loading can modify the apparent position the load is inserted at, because the linear loading wire does have some common mode current. That can actually move the effective location of the loading insertion point from the actual location of the insulator when linear loading is used. If the lumped load is placed at the same effective location, using the same size wire on almost any form factor, Q and efficiency will increase.

I can't imagine a good case for linear loading, unless the insulator cannot be placed at a good location or the linear load decouples the element to make it work an extra band.

73 Tom
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KE2TR
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« Reply #7 on: January 11, 2012, 08:04:33 PM »

Tom I am not trying to dis credit what you state on your site, I am just state that unless you use HiQ coil designs coil loading will end up with allot more loss and in most cases more so than LL. In 99 I had a F12 LL 3 element 40ty at 72 ft here in the burbs on LI, its was the second one I had bought from them cause the 1st one got damadged with a loss of a tower in a microburst. At the time I had the second antenna up there were a few station that I had made comps with, one was Arnold W2HCW the otheres were some other locals that had up the KLM 4 element big sticker. Arnolds beam was full size 40+ft boom and he had a water shoot qth which had salt water around most of the paths from JA to deep south plus the antenna was 100ft on a hill above that salt water marsh plus up around 80ft on a bertha tower, he had what some who have seen it a qth of life, it was worth maybe 4-6db over most qths here on LI just the way it would model out. I had many LP JA/VK/ZL contact with him with the 3 el F12 which I did change some of the LL on with heavier al wire plus used longer spreader that held the LL sections up so it would also decrease side to side movment on the elements. Now you can model it to death but on LP signal Arnold never had me buy more than 6db and most times its was more like 3 to 4 db and the 4el KLM I would work the station in and outa the pileup before they had a sniff. Arnold qth was awsome and to come that close with that station to me was dam good plus I had that beam at only 74ft. You can say all you want and I am sure your designs are right on the money if the coil loading that you spec will work better but from were I stand as far as real world experiances I dont fully agree. That station I built on a small qth(1/3rd acer plot) on two crank ups with 9 beams had taken 1st place CQWW M/S in 96,97 and 99 and the 40mtr scope was nothing to snease at, we were up against guys with larger statcks on 40ty. Yes LI is a great radio location if you can put up the the NY attitude but I use Arnolds comparo as another LI station which in its own right back then was world class. LL done right does work and the losses were from what I have seen not all that bad, you make it sound like a dummy load, well that was one great dummy load.
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G3UUR
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« Reply #8 on: January 12, 2012, 03:57:21 AM »

The same size wire in linear loading, at the same effective location in the element, will always be less efficient than a coil of the same wire.

This is because the linear loading really is just an inductor all stretched out of shape. We lose the advanatage of magnetic  coupling increasing inductance when we make it linear, and that requires more wire for the same inductance. It also adds more shunt capacitance, and that increases circulating currents.

There is no electrical advantage to linear loading over the same wire in a coil. Q is not even close.

Tom,

You have some very odd ideas about how things work. Your comparisons of inductive and linear loading are an ill-considered way of looking at the differences. Saving a bit of wire that’s small compared with the overall length isn’t everything! You don’t seem to be aware of the other effects that come into play. Magnetic coupling not only increases inductance. Unfortunately, it also restricts where the electrons can easily flow - the proximity effect. That increases the RF resistance far more than any saving in wire length.

If you take a half-sized 160m dipole as an example, it’s easy to see where you’re going wrong. Say the two legs are both 64 feet long and we place the loading coils in the centre of each leg, 32 feet from the ends. Coils of 80uH inductance will give resonance somewhere around 1.9MHz. It doesn’t matter exactly where for the sake of this discussion.

For convenience, let’s say the coil is made out of the same 12g wire as the antenna and we can achieve a Q of 300 with the most convenient coil geometry. That means the loss resistance is 3.18 ohms in each coil. If we change the dimensions of the coil we might be able to push that Q up to 500, but the loss resistance is still 1.9 ohms. Sure the total length of wire used in a coil of the right inductance is less than it would be for linear loading, but the length for the latter would only have to be 10 to 15% more than the extra bits of wire that are required to make the antenna full size. The loss resistance of the linear loading wire would be around 1.1 ohm because it doesn’t suffer badly from the proximity effect, while coiled inductances do. You’d have to achieve a Q of over 800 to get comparable efficiency and you’re not going to do that with 12g wire and any convenient coil geometry. Therefore, linear loading is pretty much always more efficient than inductive loading.

In addition, the circulating currents you refer to are a natural consequence of the lower characteristic impedance of the linear loading section and do no harm. In fact, they are very useful because the increased capacitance and lower inductance per unit length of the linear loading section, lower Z0, mean that the half-sized antenna will have a broader bandwidth than an inductively loaded one without sacrificing any efficiency because the reactance doesn’t accumulate anywhere near as fast as you move away from the resonant frequency.

Linear loading, if applied at the same point in the antenna as inductive loading, will usually perform better in terms of both efficiency and bandwidth. That's the theory and I've found it works in practice.

73, Dave.
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AC4RD
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« Reply #9 on: January 12, 2012, 04:09:26 AM »

... will always be less efficient than a coil of the same wire.

Tom,
You have some very odd ideas about how things work.

That may be.  And I mean no disrespect when I say this, either.  But Tom's "odd ideas" are greatly respected in the ham community, and he is considered an expert in many areas of antenna/balun/feedline engineering.  :-)
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N3OX
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« Reply #10 on: January 12, 2012, 08:01:31 AM »

Coils of 80uH inductance will give resonance somewhere around 1.9MHz. It doesn’t matter exactly where for the sake of this discussion.

For convenience, let’s say the coil is made out of the same 12g wire as the antenna and we can achieve a Q of 300 with the most convenient coil geometry. That means the loss resistance is 3.18 ohms in each coil. If we change the dimensions of the coil we might be able to push that Q up to 500, but the loss resistance is still 1.9 ohms.

Seems reasonable.

Quote
Sure the total length of wire used in a coil of the right inductance is less than it would be for linear loading, but the length for the latter would only have to be 10 to 15% more than the extra bits of wire that are required to make the antenna full size. The loss resistance of the linear loading wire would be around 1.1 ohm because it doesn’t suffer badly from the proximity effect, while coiled inductances do.

That's not a complete analysis.  The general analysis is very complex, but there are some easy examples to think about. Let's look at a concrete example where we directly replace the coil with a wide spaced (to avoid proximity effect) inductive transmission line stub... you need about +j900 to resonate the antenna in your example.  That can be achieved with 12 AWG wire using a 30cm spaced stub (Zo=690) about 53 degrees long.  The input impedance of that stub calculated using VK1OD's two wire calculator is 5.9+j908.  Using EZNEC (a bit different because of realistic end wires) I get 5.5+j870.  In either case Q is just 150 and the inserted loss resistance in each side is nearly 6 ohms.

That's also adding a lot more wire than you suggested, for what it's worth.  

This is just one example, but part of the problem with discussing "linear loading" in a general sense is that it is poorly defined what we're talking about.  Running a transmission line stub away from the antenna at 90 degrees perhaps doesn't count... probably zig-zagging back and forth at the antenna is more "correctly" called "linear loading."  But what about just running the ends of the antenna off at 90 degrees when you run out of space?  Is that "linear loading?"

Do you have a specific example of a good linear loaded antenna that uses about 15% more wire than  that we could analyze?

I  put together a well-converged model of a dipole resonant on 1.82MHz with "65 foot legs" that has about 4 ohms loss resistance:

http://n3ox.net/files/160_LL_dipole.jpg

http://n3ox.net/files/eznec/160m_130ft_LL_dipole.EZ

It's about 71% efficient, which is fine, even good (though it's still using way more wire than 15% more than the stretched-out antenna).  At first blush that seems about equal to doing the same thing with a straight #12 wire, mid-loading with lumped loads of Q = 300 and maybe 900-1000 ohms reactance  but that analysis is complicated by the fact that this antenna is two feet tall, and the "location" of the loading is complicated.  The common-mode (antenna-mode) currents are flowing on a wide structure... if you built a "planar cage" dipole with two foot high rectangular panels loaded with lumped inductors m what would happen?  How much inductance would you need?  The antenna structure is NOT a straight #12 wire, and needs a lot less reactance.

Like this antenna:

http://n3ox.net/files/160_wide_dipole.jpg

Instead of the +j1130 ohms needed to resonate a #12 straight wire, that needs just 650 ohms reactance at the feedpoint to resonate it for an additional loss resistance of 2.2 ohms from Q=300 coils, and more like 85% efficiency.  If the linear loaded model antenna I showed is more or less this "plane cage" antenna with linear loading inductors inserted in series at the feedpoint, then those inductors are closer to Q=150 like the transmission line stub I talked about first.

Furthermore, the MORE efficient "planar cage with feedpoint loading" antenna has 35kHz 2:1 SWR bandwidth compared to 17kHz for the LESS efficient linear loaded antenna.

One of the problems with linear loading analysis is that making the "circuit diagram" for the antenna is more of a challenge than in most cases.  I think that leads to apples-to-oranges analysis and we need to be careful.   In the M2 antenna that VE7RF pointed out, the analysis is easy.  The truss support/loading stub is a well defined object that can be analyzed easily as a transmission line stub.

But in most linear loaded antennas that is not the case.  The "circuit diagram" for the antenna I modeled is quite complex with mutually coupled stubs sharing wires.  In this case, the model handles it just fine but that's not always the case... and assuming that the "comparison" antenna is a single wire of the length of the linear loaded antenna isn't really reasonable.

PRACTICALLY speaking it's not going to matter for half-size 160m dipoles so we need to be careful about how performance is tested.  It seems to me that the linear loaded antenna I posted is about 3/4 dB worse than a coil-and-fat-conductor-loaded equivalent.  3/4 a dB is nearly unmeasurable, and certainly won't show up on the air.  The radiation resistance of a half size antenna is still quite high, and Q=150 vs Q=300, if that's indeed correct, is not a big deal.  But if you're trying to get the most out of an antenna with significantly lower Rrad, the loading losses become more problematic.

==============

If you've got a favorite linear loaded antenna that's optimized for efficiency I'd be happy to talk about it instead, but the couple of examples I tried seem to suggest that coils have an edge.  
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W8JI
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« Reply #11 on: January 12, 2012, 09:35:10 AM »

Quote
Tom,

You have some very odd ideas about how things work.


So do you, Dave. Very odd.

Doesn't it sound nice, to talk that way?

Quote
Your comparisons of inductive and linear loading are an ill-considered way of looking at the differences. Saving a bit of wire that’s small compared with the overall length isn’t everything!


The difference in the amount of wire, and the effective Q, can be signicant. Look at the Q of stubs compared to lumped inductors on lower bands.

Quote
You don’t seem to be aware of the other effects that come into play. Magnetic coupling not only increases inductance. Unfortunately, it also restricts where the electrons can easily flow - the proximity effect. That increases the RF resistance far more than any saving in wire length.


You don't seem to be aware of things, also. Isn't it nice to talk that way to people? Doesn't it sound professional?

Even the stub has proximity effects.

Quote
If you take a half-sized 160m dipole as an example, it’s easy to see where you’re going wrong. Say the two legs are both 64 feet long and we place the loading coils in the centre of each leg, 32 feet from the ends. Coils of 80uH inductance will give resonance somewhere around 1.9MHz. It doesn’t matter exactly where for the sake of this discussion.

It is also easy to see where you are wrong.

Quote
For convenience, let’s say the coil is made out of the same 12g wire as the antenna and we can achieve a Q of 300 with the most convenient coil geometry. That means the loss resistance is 3.18 ohms in each coil. If we change the dimensions of the coil we might be able to push that Q up to 500, but the loss resistance is still 1.9 ohms. Sure the total length of wire used in a coil of the right inductance is less than it would be for linear loading, but the length for the latter would only have to be 10 to 15% more than the extra bits of wire that are required to make the antenna full size. The loss resistance of the linear loading wire would be around 1.1 ohm because it doesn’t suffer badly from the proximity effect, while coiled inductances do. You’d have to achieve a Q of over 800 to get comparable efficiency and you’re not going to do that with 12g wire and any convenient coil geometry. Therefore, linear loading is pretty much always more efficient than inductive loading.

Not true, and field strength measurements show it.

Look at data for KLM linear loaded beams as modified to lumped loading, especially on lower bands.

A typical 12 gauge stub on 160 meters, for the equivalent of 900 ohms, has to be 80 feet long and has a Q of about 150. It has about 6 ohms loss resistance.

Quote
Linear loading, if applied at the same point in the antenna as inductive loading, will usually perform better in terms of both efficiency and bandwidth. That's the theory and I've found it works in practice.

There is nothing that supports such claims.

The best we hope for is even performance, but on lower bands where the linear loading stub gets long, losses are significantly larger than with lumped inductors.

73 Tom
« Last Edit: January 12, 2012, 09:50:17 AM by W8JI » Logged
G3UUR
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« Reply #12 on: January 12, 2012, 11:21:54 AM »

So do you, Dave. Very odd.

Doesn't it sound nice, to talk that way?

I admit I have some odd radio ideas on occasions, but that's spawned things like QER and Acromorphic filters and a few other things besides.

What's nice? What I said is honest and too the point. It's a good deal more reasonable and civilised than repeatedly insulting and criticising Warren Bruene's work and articles behind his back for no other good reason than you've misunderstood what he's written. You've been proven wrong on several occasions now in various threads and have made no public apology too him. In your parlance, "that's not nice!" In fact, your attacks on him are disgraceful and you should be ashamed of your outbursts against him. I don't know what you have against him, but whatever it is doesn't justify attacking his technical credibility behind his back.

Quote
The difference in the amount of wire, and the effective Q, can be signicant. Look at the Q of stubs compared to lumped inductors on lower bands.

Who's talking about stubs? Linear loading has many variations including lumped or distributed, and these can be parallel or orthogonal to the main antenna. Practical examples I've used in the past of both distributed and lumped using 3-inch spacing seem to indicate quite a small proximity effect and a need for around 10% extra length.

Quote
Not true, and field strength measurements show it.

Look at data for KLM linear loaded beams as modified to lumped loading, especially on lower bands.

And where's that evidence?

Quote
The best we hope for is even performance, but on lower bands where the linear loading stub gets long, losses are significantly larger than with lumped inductors.

A greater bandwidth for the linear-loaded case doesn't equate to even performance in my book, Tom. As far as one being more efficient than the other is concerned, on 160m with realistic light-weight coils it is difficult see any improvement in efficiency, but the difference in bandwidth is very evident.   

73, Dave.
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G3TXQ
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« Reply #13 on: January 12, 2012, 12:23:42 PM »

If you've got a favorite linear loaded antenna that's optimized for efficiency I'd be happy to talk about it instead, but the couple of examples I tried seem to suggest that coils have an edge.
Dan,

That's how my modelled results also turned out - the coil-loaded version was marginally more efficient, and the VSWR bandwidths were almost identical. The coil-loaded version had significantly higher feedpoint resistance: 33 Ohms vs 15 Ohms.

@Dave: what linear-loaded configuration were you proposing which performs better in terms of both efficiency and bandwidth?

73,
Steve G3TXQ
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W8JI
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« Reply #14 on: January 12, 2012, 01:24:06 PM »

Dave, there is a difference between being direct and calling people names.

We learned this in grade K, before being allowed to attend school full days.

The system over there must be good, because Steve and Peter G3RZP have no problems with social skills.

73 Tom  

« Last Edit: January 12, 2012, 03:28:04 PM by W8JI » Logged
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