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Author Topic: 2 element 75m Ham Stick Beam  (Read 512 times)

KF4FDS

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2 element 75m Ham Stick Beam
« on: March 24, 2023, 11:14:44 AM »

Has anyone ever thought of making a 2 element 75m Ham Stick Beam?

I'm thinking with close element spacing of 12-15ft from a reflector might add some gain to a rotatable dipole. 
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K1VSK

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Re: 2 element 75m Ham Stick Beam
« Reply #1 on: March 24, 2023, 11:39:03 AM »

I would model it first at whatever height you will have to see first how it might work. Intuitively, it probably won’t, at least not well enough to bother.
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W6MK

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Re: 2 element 75m Ham Stick Beam
« Reply #2 on: March 24, 2023, 11:43:06 AM »

From an online discussion of hamsticks for the lower HF bands:
"A 75m hamstick is little better than a dummy load having a radiation
resistance of maybe 0.5 ohm and an efficiency in the
ballpark of 1%, i.e. 100 watts in, 1 watt out."
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KL7CW

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Re: 2 element 75m Ham Stick Beam
« Reply #3 on: March 24, 2023, 12:24:25 PM »

Keep on thinking about various antenna configurations.  Thinking "outside the box" for solutions is a good idea, and some will probably work.  All short antennas have significant losses, so as was stated, the efficiency of a ham stick dipole probably is something like 1 %.  I modeled many Yagi antennas, and unless they were a reasonable size (say something like half size + or -) there was little or no advantage of adding a short (lossy) director or reflector.  Unless the antenna is quite high (say well over 60 feet) on 80 meters, it will be mostly a cloud warmer...it will work a little for close in QSO's (possibly) and not be very directional.  I think you would be better off using your resources to get the best (biggest) dipole up as high as you can.  Any hamstick or similar small antenna will have a very narrow bandwidth, on 80 meters (perhaps less than 10 KHz ???, you can research this.
     Your idea MIGHT work as a RX antenna, however a small terminated loop on a rotator, with a good preamp at the antenna, will have useful directional characteristics, such as a deep null, which may be several, or many S units deep, which could null out some RX RFI.  Size of loop, perhaps something like 2 feet by 3 feet.  I doubt that the hamstick antenna would match this.  The loop can be broad band and work all the way from the BCB band up into the mid HF bands, (or further). 
     Keep on investigating your ideas....many of my early ideas flopped, but eventually some paid off.  Investigate all options, end fed wires, OCF antennas, attic antennas, verticals (trap and otherwise), and even TX loops.  I am sure something will be better than a hamstick on the low HF bands.                              Rick  KL7CW
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W1VT

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Re: 2 element 75m Ham Stick Beam
« Reply #4 on: March 24, 2023, 12:30:29 PM »

A proven approach is to use a flag or two flags with phasing on the low bands. 
https://www.antennasbyn6lf.com/2016/12/comments-on-the-wd8dsb-bow-tie-rx-.html

Two things to watch out for are common mode noise and preamp overloading.
Unless you are careful noise from your station ground can travel along the coax shield of the receive antenna and make it all the way to the feedpoint.
Today's transistors have such high gain bandwidth products that cheap devices used in a low band preamp could overload on microwave cell phone signals.  Microwave shielding and lead decoupling may be necessary if you are near a cell phone tower.
« Last Edit: March 24, 2023, 12:35:07 PM by W1VT »
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WB6BYU

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Re: 2 element 75m Ham Stick Beam
« Reply #5 on: March 24, 2023, 12:56:45 PM »

Quote from: KF4FDS

I'm thinking with close element spacing of 12-15ft from a reflector might add some gain to a rotatable dipole.



Probably not at such close spacing.

While you can shrink the length of Yagi elements
(up to a point) and still get useful gain, you
can’t get away with reducing the spacing much.

Such a close-spaced beam would have a very
low feedpoint, which means higher currents,
and even higher losses in the already lossy
elements.  That’s going to eat up any gain that
you might achieve.

Another issue is bandwidth:  the Hamsticks
have very touchy adjustment, and trying to
get a reflector (or Director, which is another
option for a 2-element yagi) tuned properly
is not a trivial matter.  You can’t just tune
it for minimum SWR and have it work well.


You certainly can get some added directivity
if you can get it tuned properly, but probably
not any significant gain.

For that, you would be better off improving
the efficiency of the dipole itself.

KH6AQ

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Re: 2 element 75m Ham Stick Beam
« Reply #6 on: March 24, 2023, 01:10:38 PM »

It can be made directive but might have less gain than a HamTenna dipole due to higher RF current through the loading coils. I can run an EZNEC model if you'd like.
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WA3SKN

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Re: 2 element 75m Ham Stick Beam
« Reply #7 on: March 24, 2023, 01:23:32 PM »

Sure!
The element with 15 ft spacing will be a director though.  And the impedance will be very low so the bandwidth will also be very narrow.
btw... the current induced in a director is "in phase" while the current in a reflector is "out of phase".
But a gain pattern will be there.

-Mike.
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N4UFO

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Re: 2 element 75m Ham Stick Beam
« Reply #8 on: March 24, 2023, 01:28:28 PM »

Small size 'compromise antennas' have been created but 20m is about the lowest frequency I have seen as a beam. A former company TGM Communications built the MQ like of mini beams... I have owned two (and original MQ-2 and a later MQ-36). They work well and are fairly efficient, but over a smaller frequency range.

Two things I would guess about a 75m hamstick beam based on my experiences with 'reduced size' antennas:

1) It would be SO narrow and so touchy on tuning, it would be nearly impossible to use in any practical way. You might get a khz or two (at most) of usable bandwidth and any change in the weather would move where that usable range was. I also owned an MFJ-1796 vertical dipole as well... and while the 20m (& retuned 30m) sections worked well, the 40m section was almost worthless. Very narrow and so inefficient it was just not practical to use. (They should really redesign that antenna for 30-10m)

2) With the loading (coil) closest to the feed (instead of the end) the efficiency would be WAY down. All of the mini antennas I have worked with had coils and cap hats on the ENDS, and not near the feedpoint. This makes them considerably more efficient as the center part 'radiates' and the end loading 'tunes'.

Bottom line, could you do it? Yeah, but the result would not be practical and therefore not very useful.
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KL7CW

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Re: 2 element 75m Ham Stick Beam
« Reply #9 on: March 24, 2023, 10:10:33 PM »

W1VT has two valid points if you want a RX antenna.  I have two orthagional electronically steered flag antennas I designed for use on the BCB, 160, and 80 meters.  A preamp is probably required at the antenna with possibly 20 to 40 dB of gain if it is small.  Preamp overload was a problem when I built several published designs, for example BCB station intermod, even though I am not near any BCB stations.  After trying to fix the designs, I just built the proven W7IUV myself, and it worked great.  Some parts are hard to source, but you can get the kit for under $50 from MTM scientific, and you do not need to design the PCB yourself.  Common mode pickup on the feedline of all kinds of "junk" will be a factor to deal with.  My design used about 250 feet of cat5e cable, and I wound and designed my transformers for I think 113 ohm balanced system. I ran the signal and all the control and power wires in this single cable.  Decoupled the cable in 3 places with I think 240-31 ferrite cores, which got rid of unwanted signal pick up on the cable.  Take care to decouple the power and all the control wires in the cable.  Not an easy project to design, build, troubleshoot, etc.  but it works.  The phasing is a bit complicated, with dozens of relays, lots of diodes, and a few dozen RJ-45 connectors.  Works great from the bottom of the BCB up through 80 meters, and occasionally is better than my rotary dipole on 40. The W7IUV preamp is supposed to be rather resistant to damage by your TX antenna.  I have had no problems with this, but I only run 100 watts on 160 and my flags are about 200 feet away from my TX vertical. Hope this info might be of interest to someone, since RX noise or  RFI is often a bigger problem than radiating a signal on the lower frequency bands.
            Rick  KL7CW
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WB6BYU

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Re: 2 element 75m Ham Stick Beam
« Reply #10 on: March 24, 2023, 10:32:35 PM »

Out of curiosity I experimented with some models to see
if I could quantify the behavior.

I started with full-sized elements using 1" diameter tubing,
with 15' spacing between them.  I managed a bit over 5 dBi
gain, compared to 2.15 dBi for a single dipole.  So let's say
3 dB gain for the close-spaced 2-element beam over a dipole.

Then I shortened the elements to 40' each and added loading
coils in the center of each side.  I assigned a Q of 100 to the
coils, which is probably overly optimistic for a Hamstick.  With
a bit of tweeking I managed to get a nice pattern, with better
F/B ratio than the full-sized elements - about 20 dB over much
of the rear quadrant.  Tuning, however, was critical:  a 2% change
in the loading coil dropped the gain by about 3 dB.

An even bigger problem was that the gain dropped to -7.3 dBi,
or about 12 dB lower than the original.  For a dipole using the
same components, it was -2.5 dB, so adding the parasitic
element dropped the gain by about 5 dB, rather than adding
gain.  It did, however, provide a directive pattern that might
be useful for reducing interference from unwanted directions.

And the situation would be worse if I shortened the elements
to 16', which is about that of a typical Hamstick dipole.


What are the issues?  And why is the efficiency so much lower
on 80m than on the higher HF bands?

The main problem is the coil design.  Yes, it has a relatively
small diameter for lower wind resistance, but it turns out that
a long, small-diameter coil is not the most efficient winding
geometry.  A larger diameter coil with fewer turns, spaced about
one wire diameter apart (rather than close-wound on the former)
gives less loss for the same inductance.

And, of course, the short antenna has a lower radiation resistance,
so any resistance in the circuit reduces efficiency by a greater amount.


But there is another physical factor as well.  All of the Hamstick-type
antennas basically use the same dimensions for the fiberglass bottom
section (that serves as a coil former) and for the top whip.  Basically
the only difference is how far down the the coil extends from the top
of the bottom section.  On 80m, the coil extends most of the length
of the bottom part of the antenna, so, rather than putting the coil
at the center of each side, it is closer to the feedpoint - more like
base loading than the center loading achieved with shorter coils on
the higher bands.  That further reduces the radiation resistance,
causing higher currents and higher losses.


I have a Hamstick dipole set for 40m and 80m that was given to
me to try to see if it could be made useful.  There were two major
issues:  it was overly finicky to tune, and the efficiency was low.

The tuning was such that we could watch the SWR swing up and
down as a branch blew in the wind several feet away.  That is
a side effect of using a short antenna, and there isn't much that
can be done about it.  But a second problem appeared when we
lowered the antenna and raised it back up again, and the
resonant frequency changed without any change to the elements.

That was because it didn't have an effective balun.  As a result,
the way that the extra coax was coiled on the ground affected
the SWR curve of the antenna.  No wonder we had fits trying to
tune it!

To improve efficiency, I have picked up a pair of mobile extension
masts, something like 48" or 54", to place between the base of
each mobile whip and the feedpoint.  That will make the antenna
longer, and move the loading coil further out on the antenna.
I haven't tried this part yet - I expect I may need to extend the
whips or add a capacity hat.  But I wouldn't be surprised if that
provides as much gain (but not directivity) as you would get by
using the proposed 2-element loaded yagi on a short boom.

W1VT

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Re: 2 element 75m Ham Stick Beam
« Reply #11 on: March 25, 2023, 04:53:05 AM »

Why are losses a problem with small Yagis?
In order for a Yagi to work the current in each element needs to be approximately the same!
Thus, you have a lot of current in each and every element you add to a dipole to turn it into a Yagi.
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N2SR

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Re: 2 element 75m Ham Stick Beam
« Reply #12 on: March 25, 2023, 05:21:37 AM »

Why does the ARRL handbook still have a 3 element 40m beam with helically wound elements?   

I'd love to know how many people have actually built that antenna. 

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WB6BYU

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Re: 2 element 75m Ham Stick Beam
« Reply #13 on: March 25, 2023, 07:50:27 AM »

Quote from: N2SR

Why does the ARRL handbook still have a 3 element 40m beam with helically wound elements?   



Probably a combination of:

(A)  Nobody has come up with a replacement
article, or measured the efficiency, or

(B)  The resulting directivity is still useful, or

(C)  It actually works well enough.

While helical loading isn’t necessarily the
most efficient option, with large enough
diameter coils, thick wire, and some spacing
between the turns it needn’t be too bad,
especially if the elements are at least half
of resonant length and the spacing between
them is close to normal.

N2SR

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Re: 2 element 75m Ham Stick Beam
« Reply #14 on: March 25, 2023, 08:14:42 AM »

Probably a combination of:

(A)  Nobody has come up with a replacement article, or measured the efficiency, or

The ARRL is notorious for continuing to print incorrect information in the antenna book. 

For instance is "non-resonant" guy wire lengths.  The published lengths have been shown to be incorrect over 20 years ago.

(B)  The resulting directivity is still useful, or

(C)  It actually works well enough.

While helical loading isn’t necessarily the most efficient option, with large enough diameter coils, thick wire, and some spacing between the turns it needn’t be too bad,
especially if the elements are at least half of resonant length and the spacing between them is close to normal.

Is it really?  I'm not good at antenna modeling (not enough time to devote to learning it, plus I have friends who can do it for me), but it should be a quick/dirty model to see if that is correct.  And if it's not correct, then why continue to publish it? 

FYI, I don't mean elements with a loading coils.  I mean literally it is helically wound elements. 


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