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 Next   Go Down
  Print  
Author Topic: 706Mk2G and Auto-Coupler in car - length of coax?  (Read 3306 times)
CHRISDX
Member

Posts: 244




Ignore
« on: March 27, 2010, 07:35:16 PM »

I'm reading K0BG's web site and saw this passage (below).  The advice seems to say not to use coax between the auto-coupler (tuner) and the antenna, and make that connection very short. Since the tuner (LDG AT-7000) is in the trunk right under my trunk-lip mount and 102" whip, I can make it "inches not feet" but what to use for the feedline from the tuner to the antenna coax or antenna mount?


From www.K0BG.com site _____________________________

Again is simple terms, a coupler is a matching network which transforms the antenna resistive value to that of the transmission line, and at the same time cancels any reactance it may have. Due in part to the large reactances we're dealing with, auto-couplers used in a mobile installation must be mounted as close to the antenna as possible. That is to say, inches, not feet!

You cannot use coax for this connection either, as even a one foot piece of coax will reduce the efficiency by 30% or more. The reason? Coax has about 25 pf of capacitance per foot. The capacitance of a typical HF antenna ranges from 20 pf to about 45 pf depending on its length and frequency of operation. Since our auto-coupled antenna is essentially a base loaded vertical, placing 25 pf to ground will shunt a large portion of the RF to ground. This interaction should not be confused with using shunt reactances to match a low Z HF antennas to a 50 ohm feed. That is a different animal altogether.
Logged
WX7G
Member

Posts: 6214




Ignore
« Reply #1 on: March 27, 2010, 08:41:49 PM »

Yes you can use a short length of coax. At HF 1' of coax can be considered a lumped component.

Don't believe everything you read at K0BG.
Logged
WX7G
Member

Posts: 6214




Ignore
« Reply #2 on: March 27, 2010, 10:13:09 PM »

The reasoning behind the 30% efficiency reduction claim is that given a whip capacitance of 25 pF and a shunt capacitance to GND of 25 pF (about 1' of coax), the current out of the matching network is doubled. More current means higher matching network losses.

My objection to citing a number (30%) is that the number depends on the matching network and the frequency. But the direction is clear: Maximize the whip capacitance (longer whip) and minimize the transmission line capacitance (shorter or higher-Z).

To reduce the capacitance of the transmission line between the tuner and the antenna use 75 ohm coax, twisted pair, or better still 300 ohm TV twin lead.
Logged
K0BG
Member

Posts: 9886


WWW

Ignore
« Reply #3 on: March 28, 2010, 06:59:43 AM »

And you can't believe everything Dave tells you either. Besides the loss issue, there is a voltage issue. Even with just one hundred watts, the output voltage can reach 10 kV. No coax will sustain that voltage level! If you doubt the premise, look at the backing insulator photo on my web site. You also might want to visit this site too: http://www.ham-kits.com/mainframe.htm

Remember too, the start of the antenna is where the coax connects to the auto coupler, not the output of the coupler. If you mount the coupler inside the vehicle, you're essentially placing part of the antenna inside too. You want to avoid doing that as both egress and ingress common mode radiation will be troublesome.

Logged

WX7G
Member

Posts: 6214




Ignore
« Reply #4 on: March 28, 2010, 07:13:44 AM »

Let's analyze a simplified model of a 9' mobile whip.

The GND loss is modeled as 10 ohms. We will analyze the antenna with and without 25 pF of base-to-GND capacitance. The frequency is 7 MHz and the whip capacitance is 25 pF.

The whip base impedance impedance is 12 + j900 ohms. The radiation resistance is 2 ohms. The whip is resonated using a 20.5 uH series inductor having a Q of 150. The inductor loss resistance is 6 ohms. The input impedance of this system is 18 +j0 ohms and the inductor current at 100 watts is 2.35 amps. The radiation efficiency is 11%.

Now let's place 25 pF (about 1' of coax) between the inductor and the whip. The resonating inductor is now 10.25 uH and has a loss resistance of 3 ohms. Because the inductor is feeding current into the 25 pF whip AND the 25 pF capacitance to GND the inductor current is higher. The efficiency of this antenna is 6%. The reduction in efficiency is due to the inductor current being over twice as high.

The antenna efficiency dropped by 45%. That is close to the 30% cited by K0BG.

To improve this situation a high Q inductor can be employed or the shunt capacitance to GND can be reduced. Since your tuner is already chosen the parameter you have to play with is the shunt capacitance. Rather than a short piece of coax you can use a single wire or perhaps a length of 300 ohm twin lead.

A schematic of this antenna model could make this more clear. Maybe eham would post an article detailing this.
Logged
WX7G
Member

Posts: 6214




Ignore
« Reply #5 on: March 28, 2010, 07:25:16 AM »

At 100 watts the original whip base voltage is 1400 volts RMS.

The whip with the 25 pF shunt C has a voltage of 1200 V RMS. Either case is fine for coax.

So we see that 'stray' base capacitance does become part of the antenna tuner. It reduces the series resonating inductance thereby allowing a shorter whip to be tuned. The tradeoff is reduced antenna efficiency given the same whip length.
Logged
CHRISDX
Member

Posts: 244




Ignore
« Reply #6 on: March 28, 2010, 07:29:24 AM »

thanks for the informative replies.  i am new at this but the more i read the better.

couple 'brass tacks' questions,

1) does that 1' section using a piece of wire bring this whip e back to the 11% efficiency?  what about the coax shield at the tuner, what is that connected to?

2) how much more or less effiocient would a screwdriver antenna be, such as the little tarheel 2 with 56" whip? why?

3) this whip is very high, and sometimes hits things. i was conmsidering lowering the height by 2ft for practical reasons.  how much would the efficiency be reduced in the same scenerio above?  11% / 6%

4) right now, with the ldg at7000 tuner, i can tune (match) down to 40m, would shortening the whip by 2' make it impossible to get a 40m match any more?

thank you all for the replies
Logged
WX7G
Member

Posts: 6214




Ignore
« Reply #7 on: March 28, 2010, 08:05:18 AM »

If your 108" whip is like the one I have it can be shortened easily. Remove the whip from the base assembly and using a file cut a portion of the bottom off.

You want to know if it will still tune on 40 meters before you cut it. Use a length of wire supported by a length of wood to see what length it will tune. Use wire having a diameter similar to the whip. That would be #10 AWG.

The 11% efficiency number I gave is based on two guesses; the effective GND loss resistance and the tuner inductor Q. Given these the efficiency of the 9' whip is 11% and the 7' whip is 7%. That is a reduction in signal of 2 dB.

Note that in my calculations I am not including the height of the antenna base above the bottom of the car. The vertical length of the car (bottom of car to base of antenna) radiates and should be included in a more precise analysis. This is why mounting an HF antenna on the roof of a vehicle is good. The vehicle adds a few feet to the antenna length.

I think of an HF mobile antenna as a vertical dipole rather than as a monopole.

The Little Tarheel II with the 56" whip has a 'current-area' of 4'. The 9' whip has a current-area of 4.5'. The two antennas have just about the same radiation resistance. I believe they will give comparable performance. The Little Tarheel is more of a base loaded antenna than a center loaded antenna.

Connecting the antenna to the tuner without coax: A single length of wire will work if the current return path is along the surface of the trunk and is a straight shot. If it isn't use two wires, one to the coax inner pin and the other to the tuner GND or the SO-239 shell. For twisted pair the characteristic impedance is about 100 ohms and the capacitance is 10 pF/ft. Space the wires a quarter inch (#18 wire) and the characteristic impedance is 275 ohms. The capacitance is 3.6 pF/ft.

Given your antenna height restriction you are working in the efficiency range of 15% max. To improve your signal the next step is an amplifier. That is what I do for better mobile DXing. It was an expensive 5 dB going from 100 watts to 300 watts.
Logged
CHRISDX
Member

Posts: 244




Ignore
« Reply #8 on: March 28, 2010, 08:35:40 AM »

To me, there is a big difference between 6% or 7% and 11% or so.  So that is why I feel getting the most outta the antenna is key, before considering the idea of an amplifier.  I did check out the Ameritron 500m and the SG500. But then I would probably have to add the cost of a divorce to that. Wink

Anyway, thanks for the replies and I would like to nail down maximizing the efficiency of this whip.

I will use non-coax as short as possible.  By the way my car is a Miata, and the Comet trunk-lip mount is in the center of the trunk lid edge just behind the rear window. The whip base is probably just over 2' off the ground.

How can I measure actual radiated power and determine efficiency with my installation?
Logged
WX7G
Member

Posts: 6214




Ignore
« Reply #9 on: March 28, 2010, 10:33:47 AM »

If maximum efficiency is what you want a top loaded antenna is the thing to use. The easiest off the shelf antenna to top load is a Hustler resonator with a DX Engineering top hat.

But the Hustler resonators are rather lossy. However, with the top hat you can use a small resonator. For 40 meters you use a 20 meters resonator.

Or, you can wind your own resonator having a Q of 300 using #14 THHN wire close wound on a PVC pipe form. Set near to resonance the tuner in the trunk will touch up the match. With the fixed resonators you have to stop and change resonators to switch bands.

A homebrew loading coil can be built that looks professional enough. If you go this route I can supply you with dimensions to build to.

Let's examine the use of a Hustler RM20 resonator on 40 meters using the DX engineering Hot Rodz top hat. Six 30" spokes will tune this to 7 MHz. The mast plus resonator height is 7' above the street (where the top hat is). The RM20 Q is 100 and the effective GND loss resistance is 10 ohms. Using NEC the radiation efficiency is 20%. This is 1.5 dB over your present installation.

Now let's build our own loading coil for the top loaded case. The coil Q is 300. The radiation efficiency is 25%. This is 1 dB over the Hustler.

A model of the 108" whip mounted on the trunk and base loaded with an inductor Q of 150 the radiation efficiency is 14%. This is your present installation.

The difference between an all out top loaded antenna and your base loaded 9' whip is 2.5 dB.

How to measure efficiency in an actual installation: We can simulate the radiation resistance of a particular installation. To determine the loss resistance we measure the actual base impedance at resonance. We also measure the VSWR bandwidth as a check. The NEC simulation then has base resistance added until the base impedance and the VSWR bandwidth agree with the measurement. We now know the efficiency. This is an indirect measurement of radiation efficiency.

Another indirect method is to use a reference antenna having a known efficiency. This can be a full size vertical having 60 radials. We would expect the all out mobile antenna to be roughly 6 dB below this reference antenna.

For your use though do we really need to know the number or do we need to design for highest efficiency? The latter will get you maximum signal for minimum effort.

Go top loaded, install an ALS-500 amp and you will have a signal equal to 100 watts to a very good full size vertical. You will be the equal of most other folks you work.
Logged
CHRISDX
Member

Posts: 244




Ignore
« Reply #10 on: March 28, 2010, 12:24:27 PM »

the hustler system is definately not going to happen becuase I desire all-band ability.  also, looking to stick with a bare whip only, nolthing else showing.

i'm not sure exactly what top-loaded means, but basically I only want to have a bare whip.  right now it is 102" and today it wouldnt go under the mcdonalds drive through.  thats why i'd like to knock a couple feet off of it maybe.  but what i'm looking for is

1) how to design for the most efficiency with the bare 102" whip using my tuner (or?)

2) what would the efficiency be with the 102" bare whip when configured for most efficiency (for example, what started this thread was shortening the feedline and using something other than coax for the feedline)

3) how to measure efficiency - if possible by measuring true radiated power (is this possible with some type of device?)

4) after knowing the above, how much would the efficiency (true radiated power) decline by lopping 2' off the whip (78").  although th suggestion to just use wire to simulate whip langths is great!

is there a way for me to skip past the calculations (for now) and measure actual radiated output?  the efficiency would then be easy to deduce by dividing whats being radiated into the radio output wattage (~100w)
Logged
AA4PB
Member

Posts: 12990




Ignore
« Reply #11 on: March 28, 2010, 02:02:55 PM »

Don't use coax - not even 1 foot! Just run a single insulated wire between the tuner output and the antenna mount. Keep the wire spaced as far away as possible from any metal parts of the vehicle in order to minimize the capacitance.

Logged
CHRISDX
Member

Posts: 244




Ignore
« Reply #12 on: March 28, 2010, 10:52:41 PM »

ok.  just 1 wire from center of so239 to base of whip.

the tuner is fed with coax from the radio is this okay?
Logged
AA4PB
Member

Posts: 12990




Ignore
« Reply #13 on: March 29, 2010, 05:19:19 AM »

Make sure the tuner is well grounded to the chassis of the vehicle (short, metal staps). Use coax between the tuner and the radio.
Logged
WX7G
Member

Posts: 6214




Ignore
« Reply #14 on: March 29, 2010, 07:35:59 AM »

As with many electrical measurements the radiated power of a mobile installation is usually done via an indirect measurement. One measures one or more parameters and infers another.

A comparison against a known reference antenna is the simplest method if you want accuracy. What you will find is that the base loaded 9' whip on your Mazda will be radiating around 10% of the power applied.

It would take some doing to have the efficiency be less than 5% or more than 20%. This bounds the efficiency in a +/-3 dB window.
Logged
Pages: [1] 2 3 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!