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Author Topic: Half-Rhombic antenna termination resistance value?  (Read 1188 times)
2X2L
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Posts: 33




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« on: September 06, 2005, 02:42:26 AM »

Hello All,

I took down my modest 1/4 wavelength 11 meter (CB) whip antenna atop a 20' mast after a few too-close-for-comfort lightning strikes this summer.

To replace it, I'm looking into putting up four switch-selectable, terminated half-rhombic antennas mounted on or just above the ground, against the walls of my house.  (The half-rhombic looks like an inverted-vee and is vertically polarized, but it is highly directional just like a regular "full" rhombic.)  At CB wavelengths, making a practical-sized half-rhombic with sides 1 wavelength long is not a problem.

However, there seems to be no standard value for the termination resistance of a half-rhombic antenna.  This web page ( http://www.anarc.org/naswa/issues/199911/lib199911.html ) says that a half-rhombic mounted above a ground plane should have a terminating load resistance of 300 ohms, while these web pages ( http://www.pacificsites.com/~brooke/OE303.shtml , http://www.flmars.org/halfrhombic.htm , http://www.sniper-training-guide.com/training-guide/000283.php , and http://www.deltagearinc.com/Communication.htm ) call for termination values anywhere from 400 ohms to 700 ohms.  I'd like to go with 300 ohms because I have a few 6:1 (50 ohm:300 ohm) QRP HF voltage baluns, but I don't want to smoke my transceiver.  Has anyone here ever used a half-rhombic antenna, and if so, what termination value worked best for you?

Many thanks in advance to anyone who can help.  --  J. Jason Wentworth  
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AA4PB
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Posts: 12701




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« Reply #1 on: September 06, 2005, 05:19:55 AM »

I don't think the resistance value is particularly critical. In fact, you can eliminate the resistor entirely and have a bi-directional antenna. If the resistor is connected between the end of the antenna and ground then the actual impedance will be dependent on the resistance of the ground and radials anyway.

The important thing about the resistor is to make sure it is a non-inductive type.
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2X2L
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Posts: 33




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« Reply #2 on: September 06, 2005, 05:30:47 AM »

Thank you.  That's what I was hoping--that the termination value isn't terribly critical.  I'll use carbon composition resistors and a ground return wire under each half-rhombic (the ground conductivity changes too much here in Alaska due to seasonal soil moisture variations).  --  Jason
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WA6BFH
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« Reply #3 on: September 06, 2005, 10:36:38 AM »


I will read the web pages that you list before I comment further but, a Half-Rhombic of 1 wavelength per leg serves no purpose. Minimum size for such an antenna is at least 3 wavelengths per leg, to provide any appreciable gain. You could do better with a 2 element Yagi!

John
Also, it is not a vertically polarized either, unless it is mounted like a “V” on its side!
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WA6BFH
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« Reply #4 on: September 06, 2005, 11:17:36 AM »


Wow, that was some interesting reading!

Let me first say that the information provided on these web sites is so flawed as to be almost of no use. They are not telling (too many) untruths but, the information is so basic it is just wrong!

I could characterize it by saying things like, if I told you that you would have “all the nutrition that you need if you ate a donut per day” -- how would you value that information? Or, if I said, “an electric golf cart will supply all and any transportation needs you might have” that would be probably a good comparative analogy.

Since you seem to like military and field oriented manuals, read “Antennas and Radio Propagation” a Department of the Army manual TM 11-666. This will give you tons of information about Rhombics and V-Beams. You should also probably have “Radio Handbook” by William I. Orr, or any of Bill’s wire antenna books. You should also get “Electronic Communication” by Robert L. Shrader.

The basic information you have now is interesting, and I’m sure you will learn a bunch of things as you contrast what it tells you against more detailed information. You really need to learn and understand the physics of the way radio communication works. I would also for these same reasons look to other frequencies than 27 MHz. For “Direct Wave” (what the ill advised often call Ground Wave) it is a poor choice for radio communication, and for Sky Wave (skip) the wavelength band is so crowded as to be nearly unusable.

73, John
PS
The terminating resistors need to be the same impedance (or nearly so) as the characteristic impedance of the V-beam or Rhombic (if you utilize such resistors). This impedance will be between 400 to 600 Ohms in most installations.
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N6AJR
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Posts: 9892




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« Reply #5 on: September 06, 2005, 12:49:25 PM »

also believe it or not , it is illegal to talk more than 250 miles on the 11 meter band. check the rules, so a directional set of antennas my actually be illegal. YMMV

put up a dipole as an inverted V , coax to a dipole 8'6" on each side.
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AA4PB
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Posts: 12701




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« Reply #6 on: September 06, 2005, 01:29:16 PM »

A difference between the characteristic impedance of the antenna and the value of the resistor will cause a lower front to back ratio and more SWR excursions with different frequencies. If the resistance matches perfectly then all of the power that reaches the far end of the antenna (that which is not raditated) will be disipated in the resistor as heat instead of being reflected back in the oposite direction. Since it is difficult to know exactly what the impedance of the antenna will be in any given location and because some radiation from the rear is not normally terribly critical I figure that the exact value of the resistor is not too critical in this application. You typically see 400 to 600 ohms so 300 ohms should not be a killer.

Actually, in this application the antenna is essentially a single band antenna so I question if it is a good idea to use a resistor at all. The resistor burns up the power that would have been radiated in the reverse direction but it doesn't add anything to the forward radiated power. If there is not some special reason why you don't want signal being radiated in both directions just put up two bi-directional antennas by eliminating the resistors.
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2X2L
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Posts: 33




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« Reply #7 on: September 06, 2005, 07:42:26 PM »

I thank all of you very much for your replies and suggestions.

I realize that a low or ground-mounted vertical half-rhombic is very much a compromise antenna at these frequencies (they'd be great for vertically-polarized ground wave propagation on 160 meters or 80 meters if one had room for them!), but I'd rather not have a tall, attractive path for lightning to come into my house.  The half-rhombics will go against the sides of my house, although I could fit a three-wavelengths-per-side one against a long wooden fence that I have.  If the shallow apex angle gives it more radiation in its side lobes, I wouldn't mind a bit.

I'll try an un-terminated bi-directional one first, using my MFJ-16010 to better match its impedance to the transceiver (the RG-8 coax runs will be quite short, so the feed line losses will be low).  --  Jason  

 
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AA4PB
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Posts: 12701




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« Reply #8 on: September 07, 2005, 05:37:11 AM »

There are ways to deal with the lightning issue without using a low compromise antenna. In fact, while the low antenna may be less succeptible to a direct hit, the long wire length may be more succeptible to currents induced from lightning hitting a tree or some other nearby object. Many hams have towers of 50 to 100 feet that have been up for years without damage, not to mention radio and TV stations and cellular towers.

Check out the Polyphaser web site for some good ideas on the proper way to do lightning protection. Very basically, it is a matter of grounding the antenna and mast and the coax cable shield just before it enters the house. These methods should be followed regardless of the type of antenna you have.
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KB4QAA
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Posts: 2295




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« Reply #9 on: September 08, 2005, 10:52:48 AM »

Your depth of interest in exotic antennas is refreshing.   You don't mention what your primary objective is, local or dx which makes it hard to give definite solutions.  Some observations:

- The 1/4 vertical is probably your best all around antenna choice.  Equal coverage in all directions, no gain though.  Most CB'ers use vertical antennas, so for local contacts you will get the best polarization.  For Dx, polarization is not really a factor.  Properly ground the pole and ant. and lightning danger is minimized.  Think of all your neigbhors with poor tv antennas who never get hit.

-  The ARRL Antenna Book, 20th Edition, 2003, chapter 13 is entirely about long wave antennas.  Figure 1 shows that you need to make a rhombic antennas in free space at least 2 wavelengths long per side to equal or better a simple dipole antenna.   Therefore, you are not gaining anything by using a half-wave rhombic.  The main advantage of a full wave rhombic is extremely narrow directionality of about 30 deg.  Though I can't find a chart a 1/2 wave rhombic is going to be much less directional, again, essentially losing any advantage over a dipole's directionality.

- Your proposed installations will further degrade performance from ideal in several ways.  Mounting on the side of a structure will detune the antenna and tend to block reception (particularly for the house sides).   If you mount horizontally on the fence, even with 2 or 3 wavelenghts, which is good, you are then shooting your gain signal straight up and down.  One side into the earth, the other straight up in space.  

- In the proposed house mountings, the vertical arrangement will mean the antenna is shooting parallel along the sides of the house, not broadsides.  But being so close to the structure will block and degrade the performance.

- In essence you would be better to off making a simple dipole antenna hung horizontally from the trees for dx; vertical or sloped at an angle for local contacts.  The higher the better. You could use your old mast with a pulley at the top.  The antenna would be only about 16 feet long.  Put a 1:1 isolation balun to limit harmonics and re-radiation from the coax.  

- Your best bet would be to put up a beam antenna on your present mast.  

- Your army references are tried and true antennas but often field expedient solutions using limited equipment (especially no coax) for broad frequency coverage, not necessarily the best performance.  

- Check out the ARRL and RSGB antenna books for real performance numbers and good antenna designs.

73,  Bill.

p.s. With a ham license you can play with all sorts of antennas on many different bands.  Smiley

 
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WB6BYU
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Posts: 13045




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« Reply #10 on: September 08, 2005, 01:17:57 PM »

I just finished browsing the sites you pointed to.  Most
are about "field expedient" antennas, so have to work
with whatever materials are at hand.  So, for example,
the termination resistor made by poking two nails into a
battery may not have a predictable resistance value.
In general, the information appears to be mostly reasonable,
though it could be misleading in places if you weren't
familiar with the context.


Specifically about half rhombic antennas: you can make them
almost any size, but the really only work like a rhombic
when they are at least 3 wavelengths on a leg.  That
would be 200 feet of wire in the antenna itself (100 feet
on each leg) with a spacing between the ends of about 160
feet for 11m.  (A good starting point is that that the
length along the groun will be one wavelength shorter
than the total length of wire.)  Apply some quick math
would show that the mid point would be around 60 feet
high, so you would still need to worry about
lightening.  Even a 1 wavelength leg at a 45 degree angle
would require a height of 25 feet.  (I'm doing the math
in my head here, so you may want to check the figures
yourself.)

Actually there are two common applications for the half
rhombic these days: they look the same, but the design
considerations are quite different.  The military antennas
you linked two are designed for VHF low-band (30 - 50 MHz)
with field radios that have a built-in antenna tuner.  
The only requirement is that the impedance be within
the range that the tuner can match.  Gain is useful, but
often directivity (reducing the signal available to the
enemy) is more important.  Performance may not be optimum
on any specific frequency, but the dimensions are such
that they can be easily remembered, the antennna should
work reasonably well over the whole band, and it should
use materials that are practically available.  Remember,
these antennas are being compared to the performance of
a whip that is probably less than a quarter wave tall
with a poor groundplane underneath, so it doesn't take
much antenna to show an improvement.

On the other hand, there are some commercial antennas
from B&W and others that look the same, but are about
100 feet long and are designed to cover the whole HF
band (3 - 30 MHz).  These are about 2 wavelengths on a
leg on 10m, and clearly much less than that on the lower
frequencies.  Here the terminating resistor absorbs much
of the transmitter power, but some still manages to get
radiated.  The main advantage of such antennas is a low
SWR over a wide frequency range - just like a dummy load,
which they do tend to resemble on the lower bands.

The terminating resistor can be chosen for best front-to-
back ratio, or for flattest SWR curve across the band.
Or to give an impedance that is easy to match, or just
because that is the value that you have handy.  The
radiation pattern (and especially the front-to-back ratio)
will vary with different resistances, but that may be
OK in your application.

So, in answer to your question, varying the terminating
resistor will change the input impedance and the radiation
pattern of the antenna.  Changing the wire lengths will
also affect these things, so if you are designing for a
specific radiation pattern you will want to optimize
both factors.

Going back to your issue with lightening, I think you
will find that a standard vertical whip or dipole will
be better, presuming it is mounted on whatever mast you
would otherwise use to support the center of the half
rhombic.  And if you don't want to have an antenna up
that high, then the half rhombic isn't a good choice.


If you want an antenna with gain and directivity that is
low to the ground, I'd suggest you try a bobtail curtain,
half-square, folded Bruce array, hentenna, or "Sky Door".  
(The latter two would be turned so the long side
is parallel to the ground.)  A quick and simple approach
would be to take a full wave loop of wire in a vertical
plane and shape it so it is half as high as it is wide
then feed it in the middle of one of the short (vertical)
sides.  This will be vertically polarized and have
maximum radiation broadside to the loop.  This would be
something like 6' high and 12 feet wide, and you may be
able to hang it from the eaves of the house, or even
arrange for it to rotate on a small frame.

I think you will find one of these antennas to be much
more suited to your needs than the half rhombic (though
I certainly understand that sometimes it is fun to play
with interesting types of antennas!)
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2X2L
Member

Posts: 33




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« Reply #11 on: September 09, 2005, 04:11:15 PM »

My main concern for terminating the antenna is a flat SWR curve, not only for transmitting but for receiving on Short Wave, especially the higher bands.  Those sizes quoted are pretty tall, *but* I do have trees that are tall enough to support the apexes of several half-rhombics.  Being in Fairbanks, Alaska, there are several directions for which such a highly directional antenna would be useful (in addition to helping null out strong local AM stations off the half-rhombics' back sides).

I'm working on my ham ticket, and a much smaller roof-mounted half-rhombic would be perfect for hitting the local 2 meter and other VHF/UHF band repeaters.  This antenna would actually work nicely *with* my metal roof (using one of the corrugated metal panels as the ground plane).  As far as lightning protection goes, there are no guarantees, but I also seldom read about hams losing their homes to antenna strikes, even when they have random wire antennas that come directly indoors, so reasonable protection appears possible.  --  Jason
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WB6BYU
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Posts: 13045




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« Reply #12 on: September 11, 2005, 10:57:55 AM »

The requirements for a receiving antenna are different
than those for transmitting.  Since the signal is going
in the opposite direction, the SWR on the cable is a
function of how well the RECEIVER INPUT matches the cable
impedance, and is independent of the antenna load (though
that does affect the power transfer efficiency to some
extent.)  But gain and efficiency are not as important
for a receiving antenna: the primary limiting factor is
the strength of the signal compared to that of the noise,
and on most of the HF bands with most receivers, the
limiting noise will be picked up by the antenna along with
the desired signal.  This means that the signal-to-noise
ratio doesn't change even when there is 10dB of loss in
the feedline.

So it sounds like the main benefit of the half-rhombic in
your situation is the directivity.  If you are having
problems with a local AM BC station overloading your
receiver, then the long wire may NOT be a good choice.
(The front-to-back ratio is frequency dependent: it may
be good at some HF frequencies but not for MF.)  The
problem is that the wire length is approaching 1/4 wavelength
at the AM BC frequency, so you will get more pickup
than if you were using a shorter wire.

Of course, if you are being bothered by a SW BC station,
the situation is different.  I remember an article by
a ham who was operating from the HCJB site - he described
how he handled all the interference problems with so much
RF power floating around... maybe someone else will
remember where that was published.

For VHF, a yagi is generally a much better choice
than a half-rhombic: more gain, better pattern, much
smaller in size, and easier to rotate.  They need not
be expensive to build, either.  Of course, sometimes it
is fun to experiment with different designs, and to tell
someone that you are using an antenna that they have
never hear of!
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KB4QAA
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Posts: 2295




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« Reply #13 on: September 11, 2005, 01:23:38 PM »

HALF RHOMBIC:  IT'S A DESSERT TOPPING, IT'S A FLOOR WAX, IT'S AN ALL-BAND-GAIN-DIRECTIONAL-ANTENNA!

;-)

I think that you are turning this antenna into everything it isn't.  A ground plane antenna over a metal roof? Nah.  Stretching it from 27Mhz to 144Mhz?  Nah.  Broadband SWR antenna needed for shortwave listening?  Nah.  Hanging a rhombic vertically? Nah.  Mounting on the side of the house?  Nah.  A gain+directional antenna at less than 3-4 wavelengths long?  Nah.  Overcoming receiver front end overload by putting up more antenna?  Nah.

If you want the easiest cost effective antennas:
- Put your 1/4 CB whip back up
- Use a cheap 1/4, 1/2 or 5/8 wave 2meter whip

You want some directionality & gain for CB:
- Put up one or more dipoles oriented the directions you want to talk.
You want best directionality & gain for CB:
- Put up a beam with rotator on a mast

Overcome receiver front end overload from local AM stations:
- Buy a better radio
- Install bandpass or high pass RF filter in coax line
(still won't improve reciever problems, though).

If you want to put up a rhombic with gain and directionality for 27MHz it needs to be mounted horizonontally high above the ground, and at least 3 wavelengths long (+90 ft) to equal or better a dipole that is only 16 ft long.  The longer the better.  

All of what you want is do-able.  You just can't get it in one antenna.  And not in a vertically mounted half rhombic.

It would be easier to give helpful suggestions if you would give specific dimensions, intended frequency and mounting plans.

Good luck & 73.  Bill  

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