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 Next   Go Down
  Print  
Author Topic: 20M Hamstick On Multiple Bands  (Read 2267 times)
K3AN
Member

Posts: 787




Ignore
« on: May 11, 2006, 12:19:42 PM »

I am moving forward in my quest to go boat mobile from my pontoon boat this summer. I bought a 20M Hamstick at a local hamfest. The price was higher than I expected, so I didn't buy the second one for 17 Meters as I had planned. I set it up on the deck of the house with a mount and a couple of radials. It was pretty easy, using an Autek RF-1 antenna analyzer, to adjust the stinger length for lowest SWR on 20M.

That got me to thinking- what if the stinger was a lot shorter? Could I use the 20M Hamstick on other bands? The Autek says with no stinger in place, the frequency of lowest SWR is around 26 MHz.

There's a "stop" inside the lower section that prevents lowering the stinger more than a couple of inches. Has anyone used cut-down stingers to enable multiple band operation with a single Hamstick?

It looks like the stinger is about the equivalent of #10 wire. I will be buying some wire and conducting more tests (cutting the wire to different lengths). I'll report back with the results here.

P.S. If you enjoy experimenting with antennas, an analyzer (Autek, MFJ, etc.) is a great tool to have.
Logged
K3AN
Member

Posts: 787




Ignore
« Reply #1 on: May 11, 2006, 12:23:16 PM »

Sorry about the duplicate post. Musta had a brain f@rt.
Logged
K5LXP
Member

Posts: 4506


WWW

Ignore
« Reply #2 on: May 11, 2006, 01:53:00 PM »

You can expect a bit lower efficiency doing it this way, since more of your antenna is loading coil.  I did a similar thing where I used a 15M hustler coil on 17 and 20M by using longer whips.  It actually worked out very well.  Maybe you could trade your 20M hamstick on a 17M one and use a longer stinger on it, rather than the shorter stinger on the 20M version.

Some info on my page:

<http://www.qsl.net/k5lxp/mobile/HustlerCoil/coil.html>

Mark K5LXP
Albuquerque, NM
Logged
K0BG
Member

Posts: 9877


WWW

Ignore
« Reply #3 on: May 11, 2006, 02:39:28 PM »

Efficiency is a forte of any spirally wound antenna, so I wouldn't worry too much. Just don't try to tune it to a different band with a "tuner".

Alan, KØBG
www.k0bg.com
Logged

K3AN
Member

Posts: 787




Ignore
« Reply #4 on: May 11, 2006, 06:37:36 PM »

Alan, why not? If the tuner is only a short run of quality coax away from the antenna (meaning very little SWR-induced loss in the coax), what's the issue? There's nothing magical about the performance of resonant antennas per se.

Bill, K3AN
Logged
K0BG
Member

Posts: 9877


WWW

Ignore
« Reply #5 on: May 11, 2006, 07:23:36 PM »

Bill, the issue is a little deeper than it appears.

First, when a mobile (or any vertical) antenna is driven significantly off resonance with an auto coupler (tuner) at its base, it essentially becomes a base-loaded antenna. Using ANY amount of coax between the tuner and the antenna shunts a large portion of the RF directly to ground. Again, we are NOT talking about antennas operated AT RESONANCE !

Further, the RF voltages encountered when you do drive it well off resonance are VERY high, and may even exceed 10KV. The coils of a spirally wound antenna like the Hamstick or Outbacker, are nothing more than enamel insulated, bare copper wire. Adding insult, their small size (18 AWG and usually less) can be easily 'arced' over which ruins the antenna. That is, if the mount doesn't arc over first.

There are antennas specifically designed for this type of use, with special base insulators and proper coil spacing to handle the RF voltages present. If you check my web site under antennas, you'll see a photo of a ballmount base insulator that arced over; not an uncommon occurrence.

While I might agree some folks get away with doing this, the only reason they do is the tuner in question is absorbing most of the RF power.

Alan, KØBG
www.k0bg.com
Logged

K3AN
Member

Posts: 787




Ignore
« Reply #6 on: May 11, 2006, 08:37:50 PM »

"Using ANY amount of coax between the tuner and the antenna shunts a large portion of the RF directly to ground."

Let's assume RG-213 feedline, which has a unity SWR loss of about 1.0 dB per 100' at 18 MHz. Let's further assume 10 feet of cable, which would have 0.1 dB unity SWR loss, between the tuner and antenna. Finally, let's assume the antenna presents a 15:1 SWR. Per the Handbook charts, the total loss in the coax is just 0.7 dB. If the SWR is 10:1, the total coax loss is just 0.5 dB. That's not a "large portion" by any stretch. If you had really low-loss coax (which was my earlier post's assumption), the fact that it's between a tuner and a non-resonant antenna will still result in very little loss in the cable, even at high SWR.

I'm only talking cable loss here.
Logged
K0BG
Member

Posts: 9877


WWW

Ignore
« Reply #7 on: May 12, 2006, 06:09:16 AM »

Bill, the matching network is BEFORE the coax. The coax looks like a capacitance to ground. In other words, the capacitance of the coax (about 25 pf per foot), and that of the antenna (about 20 pf to 45 pf depending on frequency, length, etc.) are in series. The feed is the junction of the two capacitors. It should be evident what happens.

Using a chunk of coax from the coupler to the antenna is a common mistake many amateurs make when using an auto coupler with a vertical (or any antenna) on frequencies FAR REMOVED from their resonant point. A good example is Icom AH-4 and an 8 foot CB whip.

If the antenna (8 foot whip in this case) were operated on 10 meters, your analysis would be correct. Operate the antenna on say 40 meters, and we're into a different ball game altogether.

Further, the voltages even with just 100 watts input, can exceed 10KV. That amount of RF voltage will poke a hole in just about any coax you can name. It's enough to arc over mounts (as I show in my antenna article on my web page), and can even start a fire! Don't laugh, this has happened to me TWICE! And I have photos to prove it.

The other common mistake when using an auto coupler to band hop with any antenna, is the lack of a good ground connection on the low side of the matching network. It must be a very low impedance at RF (it wouldn't have to be at DC ground). A simple wire won't do the job if efficiency is a by word. And it has to be, as this combination is essentially a base loaded antenna.

Band hopping with any monoband loaded antenna is a gamble, and doing it with a Hamstick or similar antenna is a prescription to destroying the antenna due to arc over.

If you wish to carry this further, send me an e-mail.

Alan, KØBG
www.k0bg.com
k0bg@plateautel.net
Logged

VE1IDX
Member

Posts: 150




Ignore
« Reply #8 on: May 12, 2006, 05:38:16 PM »

On my installation my 20m Hamstick happens to also be resonant at 50.175 MHz,just about right for 6m without switching antennas.The bandwidth is narrow on 6m but it will alow operation on 50.100-50.250 without a tuner and I have made several contacts with it.It's great,cruise 20m when 6m is closed and when it opens jump up without changing antennas.  Smiley
Logged
K0BG
Member

Posts: 9877


WWW

Ignore
« Reply #9 on: May 13, 2006, 05:29:55 AM »

Garth, that too is a different ball game. The Hamstick, and similar antennas (and almost all mobile HF ones too), have their self resonance point just a few KHz above the frequency they are tuned to. Unlike an unloaded antenna (40 meter dipole used as a 15 meter one for example), above the resonant point the antenna starts looking like a capacitor, not an inductor. Since there are so many variables, it is hard to model one with EZNEC, but think about this.

Forgetting about self resonance for a moment, if the antenna were tuned to 14.25 MHz, the third harmonic (where the antenna would again be close to resonance) would be 42.75 MHz, not 50.125 MHz. The SWR would be very high, and capacitive reactance high enough that some form of impedance matching would be required.

What is happening is, the self resonance of the antenna is effectively removing part of the antenna, similar to a multiband trap in a dipole. There is more to it to be sure, such as stray capacitance.

This exemplifies another common misconception (assumption if you please); to wit: if an antenna presents a good match, the antenna must be working fine business. Nothing could be further from the truth! Again, just because you made contacts with the antenna, has little or no bearing on it efficiency, or it suitability.

Alan, KØBG
www.k0bg.com
Logged

W3LK
Member

Posts: 5639




Ignore
« Reply #10 on: May 13, 2006, 08:09:03 AM »

Just to back up Alan's thoughts ...

I did this very thing a few years back with a 20m Hamstick and an Alinco auto-coupler. It worked, after a fashion, on 40m, not at all on 15m, VERY poorly on 80m and in about five months I blew the board in the coupler with no warning. I had three feet of coax between the coupler and the antenna mount.

Got rid of that mess and got a real multi-band mobile antenna - a High Sierra 1800Pro.

73,

Lon - W3LK
Baltimore, Maryland
Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
AA4PB
Member

Posts: 12890




Ignore
« Reply #11 on: May 13, 2006, 08:54:14 AM »

Alan is correct. When you use coax feed on a whip antenna operated far from resonance, the issue is NOT the loss in the coax due to the high SWR. The issue is that the shunt capacitance of the coax is quite large when compared to the capacitance of the short whip antenna. Think of it in terms of two parallel resistors. If you have a 1000 ohm load resistor (the antenna capacitive reactance) and you parallel it with a 100 ohm resistor (the coax capacitive reactance), where does most if the current flow? In the 100 ohm resistor.
Logged
K3AN
Member

Posts: 787




Ignore
« Reply #12 on: May 14, 2006, 11:32:44 AM »

"Unlike an unloaded antenna (40 meter dipole used as a 15 meter one for example), above the resonant point the antenna starts looking like a capacitor, not an inductor."

Actually it's the other way around.

As you go above the antenna's resonant frequency, the antenna starts looking inductive. One well-known way to achieve a 50 ohm impedance for 80 and 160 Meter verticals is to make them somewhat longer than a quarter wave. That means you're now operating it above its naturally resonant point. You make the radiator long enough to increase the resistive component of the impedance to 50 Ohms. Doing so also adds an inductive component. A series capacitor at the base is then used to tune out the inductive component.

As you go below the resonant frequency and the antenna becomes "too short," you're operating it below its resonant point. Its reactive component is capacitive, and you need a series inductor to tune out the reactance. This is the case with most mobile antennas below 28 MHz.

This is true whether an antenna is loaded or unloaded. A 7 MHz wire dipole operated at 10.1 MHz will have an inductive reactance. However, at 18.1 MHz, it will have a capacitive reactance. At the 3/4 wave freqency (21 MHz), the reactance is once again zero and the antenna is resonant.
Logged
K3AN
Member

Posts: 787




Ignore
« Reply #13 on: May 14, 2006, 12:04:08 PM »

"When you use coax feed on a whip antenna operated far from resonance, the issue is NOT the loss in the coax due to the high SWR. The issue is that the shunt capacitance of the coax is quite large when compared to the capacitance of the short whip antenna."

Now there's a novel theory. Unfortunately it's not backed up in any engineering text. Coax loss due to high SWR is the SAME whether the reactive component is inductive or capacitive or purely resistive. You simply cannot treat the distributed capacitance of the coaxial cable as if it were a lumped constant.

Try this. Connect a 50 Ohm dummy load using a very short length of coax and measure the SWR at 14 MHz. It should be very close to 1:1. Now wire a 1000 pf (.001 uf) capacitor across the dummy load. You will find the SWR is now very high (above 6:1). Remove the capacitor and now connect the dummy load through about 35 feet of coax. That much coax has about 1000 pf of distributed capacitance (30.5 pf/ft x 35 ft = 1067 pf). Well look at that! The SWR is 1:1. Looks like the distributed capacitance isn't having any effect.
Logged
AA4PB
Member

Posts: 12890




Ignore
« Reply #14 on: May 14, 2006, 03:58:27 PM »

On the other hand, take a look at a typical AM band car radio antenna. The short whip feed impedance is very high. What happens if you use a 6-foot length of RG58 for the feed line? Not much signal. The loss in 6-foot of coax can't be that high at 1 Mhz. They use special high impedance, low capacitance cable for these antennas.

I submit that when using a whip antenna that is a small fraction of a wavelength on the operating frequency then the lumped capacitance of the coax is indeed the issue, not the loss due to SWR. Every mfg of a tuner designed to work with such an antenna will tell you NOT to use coax cable between their tuner and the antenna.

If you want a further experiment, tune up such an antenna system and bring your hand close to the base of the whip. You will see the received signal drop by many dB as the capacitance between your hand and the antenna comes into play. This is the same reason that such a whip antenna should not be mounted close to and parallel to metal body parts on the vehicle.
Logged
Pages: [1] 2 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!