Miniature self-supporting HF antennas

[David Gilbert]

Exactly, although of course a coil at the very top sees no current so
doesn't do any good there unless there is some capacitive loading above it.

Dave  AB7E


On 7/20/2015 2:44 PM, Jim Brown wrote:

> On Mon,7/20/2015 1:56 PM, David Gilbert wrote:
>> It all depends on the rest of the antenna, and yes, a very short
>> antenna with a crummy coil in the wrong place is going to suck.  But
>> some of the best antennas on the market right now use coil loading
>> very effectively.
>
> There was an excellent piece in QEX a year or two ago devoted to the
> design of short loaded antennas. It was published in two parts -- one
> dealt with measurement, the other with studying the effect of the
> position of the loading coil.
>
> The executive summary -- the part of the antenna carrying the greatest
> current does the most radiating, and for most short antennas, that's
> the part of the antenna closest to the feedpoint. The current
> distribution depends on the electrical length, including that coil. A
> loading coil near the feedpoint seriously degrades the radiation
> efficiency of the antenna, because the current maxima is in the coil,
> but the coil doesn't radiate! SO -- loading should be as far as
> possible from the feedpoint! All of this was borne out by the
> measurments.
>
> 73, Jim K9YC
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[Michael Babineau-2]

If you you are looking for a portable self-supporting antenna that is multi band and functions well, independent of terrain,
it is really hard to beat a portable magnetic loop like the Alex Loop.

I have used Magnetic Loops for at least the past 10 years (I have an MFJ and an Alex Loop  and I have built a few homebrew ones for portable use)
and I can say that the only real compromise when using a well built 1 meter diameter loop on 20m through 10m is bandwidth.  Efficiency on the
higher bands is good to excellent (usually ranging from about 30% up to about 90% on 10m).  The efficiency does  suffer
when pushing a loop this small to cover 30m and 40m but it isn’t so bad as to make it impossible to make QSOs at the 5w to 10w
level, even on SSB.

The problem with 1/4 wave verticals (including loaded ones) is that they rely on radials / earth to complete the other half of the antenna
and unless you want to spend a lot of time deploying radials (which I think  defeats the whole purpose of this thread) then results are going to
vary a lot depending on the soil conductivity. I think that most Hams would be quite surprised at how inefficient a short vertical with
few radials really is, especially over rocky terrain. Using them near salt water is whole different story.

Horizontal antennas require supports, so they are by definition not self-supporting. On most of the HF bands reasonable height is required
to achieve any kind of half decent take-off angles for DX.  Over poor soil you don’t have the efficiency issues you do with verticals.
Moxon in his excellent book "HF Antennas for all Locations”,  suggests that if you are on a mountain/hill you can use the terrain to your
advantage by mounting a low inverted-vee doublet, partway down the hill in the direction you want to work. This will significantly lower
the take-off angle and can be quite effective for working DX in this sort of environment … but it still requires a support and this only works
on a hill.

So what does a well constructed 1 meter diameter Magnetic Loop give you  :

PROs

- continuous coverage from 40m through 10m with a very close match to 50 ohms (SWR under 1.5:1 usually at resonance) on all bands
- good efficiency from 20m through 10m and reduced, but usable, efficiency on 30m and 40m (assuming proper design and construction of the loop)
- both high angle (useful for close-in NVIS contacts on 40m) and low angle radiation for DX on higher bands (note that the radiation pattern is a donut standing on its end)
- no need for an antenna tuner (in fact it is not recommended; all tuning should be done by adjusting the capacitor on the loop)
- at heights in excess of 1 radius (i.e. 1.5 feet from the bottom of the loop) very good ground independence, which makes tuning predictable regardless of terrain.
- self contained, self supporting (with small tripod), compact and lightweight making it quite portable
- directionality. A magnetic loop has bidirectional radiation pattern with lobes in the plane of the loop and a fairly deep null broadside
- very quiet antenna on receive and from my experiences even a slight rotation of the loop can drastically reduce nearby electrical interference
- 5 minute (or less) setup and takedown time
- fairly low wind resistance (you will realize the importance of this if you have ever operated on the top of a mountain ! )

CONs

- mediocre performance on 40m / 30m  (for a 1 meter diameter loop)
- narrow bandwidth (typically around 10 Khz 2:1 SWR bandwidth when tuned to resonance on a given band)  This varies from band to band but
  you can expect a wider bandwidth on the higher bands and narrower bandwidth on the lower bands.  This means that sitting on a frequency and calling CQ
  works great, while S&P is a little more work as it requires frequent tweaking of the loop capacitor to resonate the antenna as there frequency changes
- requires a means of either measuring SWR or Field Strength to adjust the tuning capacitor to resonate the loop for lowest SWR.
- need to consider RF exposure and limit power to 5W to 10W for a manually tuned loop. Also the operator should not be closer than about 1 meter from
the loop during TX at these power levels (I personally try to always sit broadside to the loop, no closer than arms length)
- possible RF burn hazard.  This is especially true for a home-brew loop with a bare radiator (i.e. copper or aluminum) and not as much of an issue for
an antenna like the Alex Loop as the coax jacket provides protection.

Observations

- manual tuning works well with a little practice and often it is possible to achieve a 1:1 SWR just tuning by ear and peaking band noise.
- for S&P, moving off frequency  a few KHz from the desired station and quick re-tweaking is pretty quick, but doesn’t work with a heavily
  occupied band (think Field Day on 20m) unless you want to be rude and TX over someone else (not recommended).
- I can’t stress the importance of proper design and construction of a loop if you are home-brewing. Everything should be soldered/braised/welded
 and you must use a split-stator or butterfly capacitor otherwise efficiency will suffer greatly.
- wide 2:1 SWR bandwidth on a home-brew loop is an indication of very poor efficiency. The trade-off with a Magnetic loop is between efficiency and bandwidth …
you can’t have both at the same time.


One area for improvement, in my opinion, would be to apply some smarts to (semi)-automatic tuning of the loop. With the technology available to us
today it should be possible to (semi)-automatically tune a magnetic loop to resonance as the user changes bands and frequencies.  
I could foresee something like a mini VNA housed inside the same box as the capacitor, being fed band/frequency data from the rig via Bluetooth (hey, I might as well think big ! ).  
Loop tuning should be possible without transmitting a signal from the rig itself.  Also because of the previously mentioned ground independence,
it should be possible to go through a calibration process to allow the loop tuning circuitry to figure out what capacitor position corresponds to a given
band and save that information as a preset for future use. This would make band to band QSYs happen a lot more quickly as it would have a good guess at a
starting point for tuning each band.  I think that  smart tuning technology would go a long way towards addressing what I see as the one major issue with a portable Magnetic
loop which is the narrow bandwidth and the need for frequent retuning.  

Michael VE3WMB / VA2NB

P.S.  The standard disclaimer,  I have no personal or financial interest in Alex Loop.






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[Wes (N7WS)]

Your deep suspicion is probably unjustified.

Certainly, examples can be found of inefficient antennas which suffer some
inefficiency because of the coils but generally, the inefficiency is due to the
need for the coils in the first place.

Any antenna satisfying what I perceive are Wayne's needs is going to be
inefficient because it is small and operated in close proximity to Mother Earth.
There is a good chance that conductor and ground losses swamp coil loss.

If one is going to compare one antenna with loading coils to another without
coils to determine efficiency then they they must be tested under the exact
conditions.  In other words, take measurements of antenna one, remove it and
replace it with antenna two and note the change. Any other comparison is
uncontrolled and suspect.

Wes  N7WS

On 7/20/2015 2:46 AM, Rick M0LEP wrote:

>> So far, I've found nothing that comes close, and there's nothing more
>> frustrating than getting to the top and then finding the antenna you
>> have is doing a poor job. I now regard anything which relies on loading
>> coils with deep suspicion. That coil's usually doing a fine job of
>> converting RF to heat. One such antenna I tested against an inverted-V
>> dipole turned out to be over 20dB down on the dipole for 40 metres.
>>
>> On Sun 19 Jul Wayne Burdick wrote:
>>> But the search for the ideal miniature HF antenna continues: something
>>> both very compact *and* highly efficient. Ideally it would break down
>>> to a length of 8" or less, do an excellent job on 20 meters and up,
>>> and earn a passing grade on 30 and/or 40 meters.
>

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[Rick M0LEP-2]

That is exactly what I did, with the help of a friend a reasonable
distance away with a reasonable S-meter, repeated antenna swapping, and
a little help from skimmers on the RBN for control. I'm pretty sure the
difference is real. I'd rather carry a lightweight telescopic pole with
the means to guy it, and use an inverted-V dipole than trust that loaded
vertical to get me contacts.

On Tue 21 Jul Wes (N7WS) wrote:
> In other words, take measurements of antenna one, remove it and
> replace it with antenna two and note the change. Any other comparison
> is uncontrolled and suspect.

--
73, Rick, M0LEP   (KX3 #3281)

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[Elecraft mailing list]

Sorry Rick,

Please let me explain,  doing a antenna substitution works only in the following situations.
1.  Both antennas are designed to have patterns similar in the direction of the receiving station or.2.  Both antennas have the same TOA to the receiving station or3.  Both antennas had the same amount of delivered power to the antenna or

And I could go on.  The test you did is a nice comfort feeling one but does not tell you why one works better that the other one.
IMHO
Mel, K6KBE

      From: Rick M0LEP <[hidden email]>
 To: [hidden email]
 Sent: Tuesday, July 21, 2015 3:08 PM
 Subject: Re: [Elecraft] Miniature self-supporting HF antennas
   
That is exactly what I did, with the help of a friend a reasonable
distance away with a reasonable S-meter, repeated antenna swapping, and
a little help from skimmers on the RBN for control. I'm pretty sure the
difference is real. I'd rather carry a lightweight telescopic pole with
the means to guy it, and use an inverted-V dipole than trust that loaded
vertical to get me contacts.

On Tue 21 Jul Wes (N7WS) wrote:
> In other words, take measurements of antenna one, remove it and
> replace it with antenna two and note the change. Any other comparison
> is uncontrolled and suspect.

--
73, Rick, M0LEP  (KX3 #3281)

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[Wes (N7WS)]

Unless I misunderstand (always a possibility) you compared a coil-loaded
vertical to a wire inverted vee.  Is this correct?

If so, you are not determining the effects of loading coil loss, you are
determining that a vertical antenna is different from a (nominally) horizontal
antenna.  It takes no experimentation to know this.


On 7/21/2015 3:08 PM, Rick M0LEP wrote:

> That is exactly what I did, with the help of a friend a reasonable
> distance away with a reasonable S-meter, repeated antenna swapping, and
> a little help from skimmers on the RBN for control. I'm pretty sure the
> difference is real. I'd rather carry a lightweight telescopic pole with
> the means to guy it, and use an inverted-V dipole than trust that loaded
> vertical to get me contacts.
>
> On Tue 21 Jul Wes (N7WS) wrote:
>> In other words, take measurements of antenna one, remove it and
>> replace it with antenna two and note the change. Any other comparison
>> is uncontrolled and suspect.

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[Rick M0LEP-2]

(Consolidating replies to a couple of messages here...)

The object of my exercise was not to measure the exact losses in the
loading coil, nor to determine why it made such a poor antenna, but to
determine which antenna would work best for the appropriate bands from a
SOTA summit.

The antennas under consideration were the loaded vertical (adjustable
for all the bands required), a link-dipole (covering bands up to 15
metres), and various mono-band elevated ground plane verticals (one for
each band from 20 metres to 10, though only 12 and 10 aren't covered by
the dipole). I'd taken the loaded vertical on a couple of activations.
One had been a reasonable activation (14 contacts on bands between 40
metres and 12 metres) and one had been a struggle (just 4 contacts,
enough for the points, in over an hour and a half of calling on HF
between 60 metres and 12 metres, with me resorting to CW for the last
contact), but on both I'd noticed the reports I was being given were
lower than I'd have expected.

My friend could only help with the lower HF bands, so I have more
confidence in those tests, and I wasn't expecting the loaded vertical to
shine when compared to the dipole, but the magnitude of the difference
was surprisingly wide. To give myself a baseline expectation I'd first
tried contacting him with my (then) main 27ft tall vertical (which had
some top-loading and a feed-point ATU), both with my main rig and the
KX3 I planned to use to test the portable antennas. Somewhere I had
detailed notes of all the numbers, but I can't find them. However, the
QSOs are also in my log, and with the main rig at 100 watts we reported
5 and 9 both ways. With the KX3 at 10 watts we reported 5 and 7 both
ways. I then took the KX3 outside to test the portable antennas, using
it at the same 10 watt level. The best QSO on the loaded vertical has me
giving him 5 and 1, and he giving me 3 and 1. On the dipole the log says
5 and 9 both ways. Some of that will be down to radiation pattern,
height above ground, and so on, but it confirmed experience from the
second summit mentioned above. Clearly, if I want to make contacts on
the lower HF bands then the dipole is a far better bet.
 
For the higher bands I had to rely on RBN and any contacts who happened
to answer, so the comparison's a bit harder to quantify exactly. (RBN
skimmers don't report every CQ, so you have to play games changing
frequencies, and waiting, in order to get enough coverage.) However, as
best I could figure, the loaded vertical was well down on the un-loaded
mono-band ones as well as on the link dipole. Again, height above ground
level will have had some effect. The loaded vertical has its own little
tripod, so isn't far off the ground. The others all rely on telescopic
fishing poles, so they're at least a few feet higher off the ground.

In all cases the loaded vertical was worst, in some cases by a very
large margin. At best (on 10 metres, which was clearly its best band) it
was only 6dB or so so down on the elevated ground-plane vertical. That
could be down to elevation, radials, the loading coil, or all three.

I'd rather over-optimistically hoped that the self-supporting loaded
vertical (total weight 1.5kgs) would do the SOTA job nicely. The dipole,
a couple of verticals for the bands the dipole doesn't cover, pegs,
guys, and a short (6 metre) telescopic pole weigh 2kgs in total.

For SOTA purposes the loaded vertical was not worth carrying despite its
(mostly) glowing reviews on eHam. Sure, I've made a few contacts with it
from SOTA summits, so it wasn't totally useless, but the alternatives I
now take are far better.

Next time I'm tempted to make a link-dipole I'll make sure it has
additional links for 10 and 12 metres so I can save myself the weight of
the elevated ground-plane verticals I usually carry for those bands.

On 21 Jul 2015 Wes N7WS wrote:
> If so, you are not determining the effects of loading coil loss

On 21 Jul 2015 Mel, K6KBE wrote:
> does not tell you why one works better that the other one.

--
73, Rick, M0LEP   (KX3 #3281)

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[Edouard Lafargue]

     To add to the small self-supporting thread - loads of great
contributions there, thanks everyone - : I travel a lot for business, and
often carry my KX3 + Alexloop which is pretty much the only kind of antenna
that I know I'll be able to use in urban areas. This setup works really
well: on my last trip to France, I was able to do lots of digital QSOs all
over Europe, as well as several countries on phone on 20m. And I also had a
couple of QSOs on 40m with French hams who could hear me fairly well in a
large part of the country - not everywhere though, 5W on a 1m diameter
magloop on 7MHz only takes you so far.

    I am in Dallas TX right now, and I spent time on 20m phone last night:
59 reports from Georgia and Montreal, CA, with a long QSO with two very
friendly hams from Canada. We even had two guys from Reunion Island join us
- one of them could not hear me, the other said he could, then band
conditions changed and everyone drowned into band noise before I could
complete the QSO... that's the life of a QRP operator.

    When I know I can operate outside, a Sotabeams fiberglass pole with
thin wire and a 9:1 Unun (http://www.aerodynes.fr/2014/05/18/peanut-balun/)
do a nice job, but they are impractical in cities, of course.

    Only circumstantial evidence, I know, and anyone can do a QRP phone QSO
around the globe if conditions are just right, but still, I have tried
quite a few 'hotel room' setups, and right now, Alexloop/KX3 is really my
favorite. Now, if only more North American hotels allowed their guests to
open their windows...

     My two last points:  1) I'm tempted to build an autotuner for the
AlexLoop, I have a couple of ideas on how to do this very simply, stay
tuned (pun intended). 2) Even on a Magloop, the PX3 is useful, and I would
like to make the PX3 battery operated as well. There is definitely room in
the box, and looking at the PCB, there is a connector inside just for that
purpose, right Wayne ?

73 de Ed W6ELA

On Sun, Jul 19, 2015 at 11:45 AM, Wayne Burdick [hidden email] [KX3] <
[hidden email]> wrote:

>
>
> Hi all,
>
> Have you found the "perfect" above-the-treeline backpacking antenna for
> use with your KX3 or other small rig? I've used everything from a 10-meter
> coat hanger whip, to a yagi that breaks down into two dozen pieces, to a
> dipole held up at the center by a willing (and tall) campmate. The variety
> (and price range) of such antennas is staggering.
>
> I've had pretty amazing results using short, base-loaded antennas on the
> higher bands--especially when conditions were good. My personal best is JA
> from W6 on 15-meter SSB, running 3 watts to a Maldol 48" whip. These
> antennas collapse and break down into just two pieces, taking very little
> space in my lightweight go-bag. This leaves room for a couple of 25' wires,
> adapters, and weights for times when there are trees available.
>
> But the search for the ideal miniature HF antenna continues: something
> both very compact *and* highly efficient. Ideally it would break down to a
> length of 8" or less, do an excellent job on 20 meters and up, and earn a
> passing grade on 30 and/or 40 meters.
>
> One other key factor, at least with the KX3/KX1/K1 genre, is to take
> maximal advantage of the rig's internal ATU. A wide-range ATU (such as the
> KXAT3) can turn a narrow-banded antenna into one that covers a full band or
> even multiple bands, within limits. One general approach is to coarse-tune
> the antenna's own inductance, then let the ATU do cleanup.
>
> Is the best antenna for backpacking a very small magnetic loop? A cleverly
> designed, center-loaded telescoping whip? A length of #30 wire lofted by a
> small helium balloon? (Or, more intriguingly, some combination of these?)
>
> I'd be interested in hearing about your antenna theories and field
> experiences, backed up by entertaining fish stories, if they aren't
> embellished to an embarrassing degree. If your supporting documentation is
> too voluminous for the forum (attached photos, etc.), feel free to email me
> directly.
>
> If anything substantive or surprising emerges, I'll do a followup posting.
>
> 73,
> Wayne
> N6KR
>
>  __._,_.___
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