Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 3 4 5 Next   Go Down
  Print  
Author Topic: Hamstick, Hi-Q or Tarheel 40 or 75  (Read 20789 times)
KF7NUA
Member

Posts: 153




Ignore
« on: November 13, 2012, 02:55:11 PM »

Is there that much difference that I could be missing?
I feel I have a better than avg mobile setup and I am wondering if I can still be missing quite a bit of TX and RX.
To give you an idea, I usually have no problem reaching any area of the US and Canada.
I have also reached Europe, Africa, Japan, So America, Australia and New Zealand when the other operator was on a big station with a strong signal and the pileup is small, it may take a while and if I get lucky I connect with them for the contact.
I am located in So. Arizona and I do this with Hamsticks, sure I know when the propagation is right anyone should be able to do it with a wet noodle.

I also have a Hi-Q 4/80 sitting in the garage that I cannot seem to find a good spot to mount on my truck without severely compromising it because of the oversize Aluminum Security cap/container mounted in the bed that is just over 8ft tall from the ground.

After looking for an alternative screwdriver, I am sure I can mount a Tarheel 40 or Tarheel 75 without any problems but will I gain any performance over the hamsticks I have been using? This is a costly move if it is a lateral move, but if it is a better move up in performance I may do it. I am usually on 20m, 40m and sometimes on 75m.

Any thoughts on this? 
Logged
WX7G
Member

Posts: 6146




Ignore
« Reply #1 on: November 13, 2012, 06:08:12 PM »

The Tarheel 40A (I have one) does not do 75 meters, so that option is out unless you do what I'm planning to do. And that is to mount a loading coil at the top of the 40A when I want to work 80 meters. I'll have it built in a week or two. It consists of a 10" long 2" PVC pipe close wound with #14 stranded THHN wire.

I've analyzed the Hamsticks (I have those too) and on 40 meters in my installation the performance advantage of the Tarheel 40A over the Hamstick is 5-6 dB based on impedance measurements and simulations.

The 40A with the additional loading coil for 75 meters will exhibit an even greater improvement over a Hamstick.

My simulations show the 40A being about 2 dB better on 40 meters than the 75A. So if you want 3 or 4 dB over the Hamstick on 40 meters the 75A is the way to go. If you want 5 or 6 dB get the 40A.

If you want to see my path to the 40A read the Little Tarheel II thread down the page.

Logged
AC4RD
Member

Posts: 1235




Ignore
« Reply #2 on: November 14, 2012, 09:19:44 AM »

I think the hamsticks and similar are best on 20m and higher frequencies.  For 40 and lower, a good screwdriver can have higher efficiency than a 'stick.   Me, since 14mHz is the lowest frequency I use in the car (15m is my favorite band and 12m is a close second when conditions are good), I used hamsticks for several years.  These days I'm using a homebrew/Hustler system that works pretty well and is a bit better than the hamsticks IMO.

But yeah, if you're interested in 40 and 75, my understanding is that you definitely might be better off with a screwdriver type antenna.  GL!
Logged
WX7G
Member

Posts: 6146




Ignore
« Reply #3 on: November 15, 2012, 06:46:53 AM »

Something interesting about the Tarheel 40A, 75A, and the Hamsticks is that only the 40A requires a shunt inductor for matching. That tells us that the 75A and Hamstick are rather lossy. I'm talking about 40 meters here.

The measured base resistance of my 40A is 16 ohms. The Hamstick is close to 50 ohms and the 75A would probably be 35 ohms or more or they would include a matching inductor. In my installation the radiation resistance of each antenna on 40 meters is roughly 4 ohms. Based on the 16, 35, and 50 ohm base impedances of the three antennas the radiation efficiencies work out to be 25%, 11%. and 8% for the 40A, 75A, and Hamstick respectively.
Logged
K0BG
Member

Posts: 9880


WWW

Ignore
« Reply #4 on: November 15, 2012, 08:23:23 AM »

I would doubt that a 40A is 16 ohms. You have at least 6 to 8 ohms of ground loss on 40 meters. The whole antenna is just 8 feet long, fully extended, so the Rr is about 1.3 ohms. That would mean the coil loss would need to be no more than 5.5 ohms. It is more like about 12 or 14. I'd believe about 25 ohms perhaps.
Logged

WX7G
Member

Posts: 6146




Ignore
« Reply #5 on: November 15, 2012, 11:47:29 AM »

Alan,

the 40A is 7.5' on 40 meters. It is mounted on the top of a 5.5' vehicle. My "antenna" is a ~12' vertical dipole consisting of the vehicle and the 40A. If you prefer to view my antenna as a ground mounted vertical it is a ~12' vertical.

That is where the roughly 4 ohm Rr comes from, the 12' antenna length. I measured the feedpoint to be 16 ohms. I will run a NEC simulation this weekend to get a more true-to-life Rr. Four ohms is a back-of-the-envelope calculation.

In each of the three cases cited - 40A, 75A, and Hamstick - the Rr is a small fraction of the measured input impedance. Therefore the relative efficiencies based on base impedance measurements (and implied base impedance of the 75A) are valid.

The 40A is 5-6 dB more efficient than the Hamstick. Is this what you would expect in this installation?  






    
« Last Edit: November 15, 2012, 12:11:47 PM by WX7G » Logged
K0BG
Member

Posts: 9880


WWW

Ignore
« Reply #6 on: November 15, 2012, 02:23:26 PM »

David, I've heard you spout that conjecture before, and it has the same merit your antenna patents have.
Logged

WX7G
Member

Posts: 6146




Ignore
« Reply #7 on: November 15, 2012, 07:15:28 PM »

Alan,

what I stated is not a conjecture, it is a hypothesis. To credibly refute my hypothesis you must use mathematics and/or simulation. Not hand waving, not sophistry, but true engineering/scientific analysis. You are invited to provide an alternative hypothesis that you must prove mathematically and/or by simulation.



« Last Edit: November 15, 2012, 07:39:21 PM by WX7G » Logged
ZENKI
Member

Posts: 960




Ignore
« Reply #8 on: November 16, 2012, 12:37:07 AM »

A hamstick with a hamstick radial seems to be doing well in the antenna shootouts on 40 meters. Why not give it try and report back. I will be trying  this configuration next summer.

If you can get within a 0.5db of a Scorpion antenna without all the heavy mounting and engineering you are doing well.  Big heavy screwdrivers are not practical for most people and nor do most people
require the performance if they just chatting within their own country.

Logged
KF7NUA
Member

Posts: 153




Ignore
« Reply #9 on: November 16, 2012, 04:58:49 AM »

A hamstick with a hamstick radial seems to be doing well in the antenna shootouts on 40 meters. Why not give it try and report back. I will be trying  this configuration next summer.

If you can get within a 0.5db of a Scorpion antenna without all the heavy mounting and engineering you are doing well.  Big heavy screwdrivers are not practical for most people and nor do most people
require the performance if they just chatting within their own country.



Yes you are right, a hamstick is a simple antenna compared to a screwdriver and I do well well with them but the problem is everyone I meet usually says that I am missing much much more signal without the screwdriver, this is why I asked.

By the way what is a hamstick radial?Huh??
Logged
KF7NUA
Member

Posts: 153




Ignore
« Reply #10 on: November 16, 2012, 05:03:30 AM »

Dave - thank you for the tech info and yes I have read your other thread, that is one reason I had chosen the Tarheel as a possibile alternative.
Logged
K0BG
Member

Posts: 9880


WWW

Ignore
« Reply #11 on: November 16, 2012, 06:08:54 AM »

If it were true, then the input to my mobile antenna would easily be over 50 ohms. While there is current flowing through the body of the vehicle it is no different than the current flowing through a radial system. The only difference is the losses in vehicle radial systems are somewhat greater. But that fact does not mean the vertical is electrically longer as a result.
Logged

WX7G
Member

Posts: 6146




Ignore
« Reply #12 on: November 16, 2012, 07:56:55 AM »

Alan,

as you know the reason a symmetrical, horizontal radial system does not contribute to the radiation resistance of a monopole is because it does not radiate. The magnetic field of opposing radials cancels.

Now picture the same monopole but having radials that slope downward at a 45 degree angle. This configuration is used to increase the radiation resistance of the antenna. The increased radiation resistance comes from the downward sloping radials radiating. While the horizontal magnetic field component of opposing downward sloping radials cancels, the vertical component does not. This antenna is referred to as a monopole but is actually a dipole.

A vehicle with a monopole mounted on top acts like the monopole having downward sloping radials. The current flowing vertically along the vehicle body leads to radiation and contributes to the radiation resistance of the antenna; the antenna consists of the vehicle and the monopole. As with the monopole having downward sloping radials it is referred to as a monopole is is actually a dipole.

A NEC simulation of this is enlightening. Set the vehicle/monopole over perfect ground and measure the resonant frequency and input impedance. Now set the vehicle/monopole in free space and measure the resonant frequency and input impedance. The change in both parameters is relatively small and one can say that the vehicle/monopole forms a largely ground-independent antenna. And as with a vertical dipole the vehicle vertical dipole incurs additional loss when placed close to lossy ground.

Where the monopole is placed on the top of the vehicle matters. When placed in the center of the roof the current flow vertically along the vehicle body is lower than when the monopole is placed at the edge of the roof. In my installation the monopole is placed close to a rear corner of the SUV. This is the best location for radiation resistance.

Working the numbers for my installation with paper-and-pencil the 16 ohm input impedance is composed of:

 3 ohms loading coil (500 ohm reactance, Q =167)
 4 ohms lossless feedpoint resistance
 9 ohms other losses (ground)
16 ohms total

What if the antenna lossless input resistance is in error? Given that it is a relatively small amount of the total input resistance the relative difference between the 40A and the Hamstick is still valid. And that is what the goal of all of this was - to improve my mobile signal on 7 MHz.  

I am able to perform calibrated field strength measurements comparing the Tarheel 40A and the Hamstick and will do this if I write an article on all of this. I expect to measure a 3-6 dB difference in my installation.

Is this what you would expect, a 3-6 dB difference at 7 MHz?

 


« Last Edit: November 16, 2012, 08:27:01 AM by WX7G » Logged
K0BG
Member

Posts: 9880


WWW

Ignore
« Reply #13 on: November 16, 2012, 10:56:36 AM »

The body of the vehicle is coupled to the surface under it. The standing waves between them is what causes the losses in the first place. And that surface is indeed horizontal. While there is radiation from the body of the vehicle, it contributes very little far field radiation.

Higher placement lowers the amount of ground loss because more of the return current flows through the body, rather than the lossier surface under it. Again, that doesn't increase the electrical length of the radiator. The electrical length, and the way the current flow through that length, determines the radiation resistance. The ground loss effects that current and its pattern, but it doesn't increase the electrical length of the radiator. To assume otherwise is a gross error.
Logged

WX7G
Member

Posts: 6146




Ignore
« Reply #14 on: November 16, 2012, 02:54:48 PM »

The far-field E-field (and H-field) produced by a wire carrying RF current is proportional to the current integrated along the length of the wire. This is called "current-area."

1. A mobile whip has RF current along its vertical length. The current-area is equal to the current integrated along the length of the wire. It produces an E-field.  

2. A vehicle body has RF current along its vertical length. The current-area is equal to the current integrated along the vertical length of the vehicle. It produces an E-field.

This is the most fundamental thing to understand about HF mobile antennas. It allows one to quantify how antenna placement affects radiation resistance.

Logged
Pages: [1] 2 3 4 5 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!