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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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  #21 (permalink)  
Old 01-14-2018, 5:19 PM
prcguy's Avatar
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If a transformer of any type is used with an autotuner right at the antenna, a 1:1 should be the only choice. On 80 and 40m the feedpoint impedance will be very low and well under 50 ohms. Above 20m there will be frequencies where it can be very high impedance.

Adding any transformer between a tuner and the antenna feedpoint is just adding loss, probably making it easier for the tuner to find a match at the cost of signal strength.
prcguy

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Hmmm...4:1 UnUn or 9:1 UnUn?
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  #22 (permalink)  
Old 01-14-2018, 5:41 PM
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I agree- without a counterpoise, using only a ground rod- you will be heating up a lot of earth ! ...
.
And no transformers!
.

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All that RF return, occuring at those 'magic nodes' of a quarter wave etc., will not be flowing thru a low loss wire matrix- but thru high loss earth. If one is lucky, the earth my have a high conductivity- but if its dry, sandy, rocky stuff- anticipate half your power gone, right there. Warm happy gophers....
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What is upsetting (well, maybe to me) is that radio Hams think they can drive a ground rod into the earth at the base of a vertical and call it "done." This is especially sad since it is entirely possible to achieve a great SWR reading, but the innocent, Unaware doesn't know that this is basically a earth warming arrangement.
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With a ground mounted vertical, the ideal is at least 120-or-so, radials, at least a quarter wavelength, streaming 360 in all directions about the base (and NOT buried deep!- remember- any return thru any earth: Bad.........) This will capture and return the majority of the field, and approach a 0 db loss. You want all the return to be Copper' ( or the metallic of your choice)-- not Mother earth. Raise the base and its counterpoise to snatch all the return before Mother chews it up....
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With a vehicle (that includes trailers etc.) such ideal counterpoises are not possible. The return path is going to miserable at best. You want as much of the return thru the vehicles metal body- why the elevated base. Yes, an HF mobile whip can show such a beautiful SWR, but half its power can be going, right off- for naught. Add in all the other factors, and it can seem hardly worth the while. My example of the 8 Mhz mobiles are a case in point- with their short whips, the efficiency is calculated at 12% compared to a perfect quarter wave over a perfect ground. But surpisingly this translates into only a few "S" units difference. If you are careful about the placement of the base on the vehicle, you can gain another decibel (if that is important- but also look to things like low loss loading coils, centre and top loading v.s base- for instance---this will effect where the maximum current flows in the vertical-- These pay big dividends.)
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Ground losses are things you can't do much about with mobile setups- but the other things can lessen the over all losses tremendously.
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Stand back from your antenna and imagine all the return paths to its base-- the more that can be eliminated in being burned up by the earth, the higher the efficiency.
.
.
...................................CF
.

Last edited by Coyote-Frostbyte; 01-14-2018 at 5:49 PM..
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  #23 (permalink)  
Old 01-14-2018, 6:23 PM
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To add even more to this, I think the OP will be better off RF wise and neighbor wise in an RV park if he uses the metallic shell of his RV as the counterpoise rather than spooling out an impossible to achieve radial system on the ground. A 30ft Airstream has a lot of ground plane available and should work fine even down to 80m.

As an example of this, I have a fairly large pickup, a Tundra double cab with a medium size Tarheel 100HP screwdriver antenna mounted behind the cab. I also run various large wire antennas at home ranging from G5RV to ZS6BKW to 80M OCFD and similar, all at 30ft height and horizontal.

I've compared my mobile setup running 500W to my home setup running 100W to distant stations 1,000 to 3,000 mi away, and on 40m the mobile is consistently around 3dB higher in signal level than the house antenna. The difference in power between the 500W mobile and 100W base is about 7dB, leaving the mobile antenna only 4dB worse than my huge base antenna 30ft up. That's not bad and a larger antenna like a 27ft vertical with a much larger counterpoise than my pickup truck will be even better.

I really think an auto tuner grounded to the Airstream feeding the 27ft vertical at roof height will make the OP real happy.
prcguy



Quote:
Originally Posted by Coyote-Frostbyte View Post
I agree- without a counterpoise, using only a ground rod- you will be heating up a lot of earth ! ...
.
And no transformers!
.

.
All that RF return, occuring at those 'magic nodes' of a quarter wave etc., will not be flowing thru a low loss wire matrix- but thru high loss earth. If one is lucky, the earth my have a high conductivity- but if its dry, sandy, rocky stuff- anticipate half your power gone, right there. Warm happy gophers....
.
What is upsetting (well, maybe to me) is that radio Hams think they can drive a ground rod into the earth at the base of a vertical and call it "done." This is especially sad since it is entirely possible to achieve a great SWR reading, but the innocent, Unaware doesn't know that this is basically a earth warming arrangement.
.
With a ground mounted vertical, the ideal is at least 120-or-so, radials, at least a quarter wavelength, streaming 360 in all directions about the base (and NOT buried deep!- remember- any return thru any earth: Bad.........) This will capture and return the majority of the field, and approach a 0 db loss. You want all the return to be Copper' ( or the metallic of your choice)-- not Mother earth. Raise the base and its counterpoise to snatch all the return before Mother chews it up....
.
With a vehicle (that includes trailers etc.) such ideal counterpoises are not possible. The return path is going to miserable at best. You want as much of the return thru the vehicles metal body- why the elevated base. Yes, an HF mobile whip can show such a beautiful SWR, but half its power can be going, right off- for naught. Add in all the other factors, and it can seem hardly worth the while. My example of the 8 Mhz mobiles are a case in point- with their short whips, the efficiency is calculated at 12% compared to a perfect quarter wave over a perfect ground. But surpisingly this translates into only a few "S" units difference. If you are careful about the placement of the base on the vehicle, you can gain another decibel (if that is important- but also look to things like low loss loading coils, centre and top loading v.s base- for instance---this will effect where the maximum current flows in the vertical-- These pay big dividends.)
.
Ground losses are things you can't do much about with mobile setups- but the other things can lessen the over all losses tremendously.
.
Stand back from your antenna and imagine all the return paths to its base-- the more that can be eliminated in being burned up by the earth, the higher the efficiency.
.
.
...................................CF
.
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  #24 (permalink)  
Old 01-15-2018, 7:21 AM
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+1 on prcguy's suggestion to ensure you are using the body of the Airstream as an RF counterpoise. Make certain the outer shield of the coax or the ground on the tuner is connected directly to the frame / body of the RV. I have an Eagle One vertical mounted on the ladder of my RV. The outer conductor of the coax is connected from the base of the vertical to a large metal bolt that is directly into the RV chassis. I used a wire brush on a power drill to get all the paint off before making the connection. The Eagle One is a stupid-simple antenna.... about 30 feet of wire inside telescoping PVC tubing. My tuner is an LDG IT-100. I've had great success with the vertical at many campgrounds on 40M-6M. Couldn't get it to tune on 80M though. So another ham and I tried to see if we could get it to load up on 80M by running about 100 feet of #14 wire from the same ground lug at the base of the antenna. We just laid it out in a straight line away from the RV.... worked fine on 80M and the other bands!
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  #25 (permalink)  
Old 01-15-2018, 12:51 PM
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Another consideration to keep in mind is one I just touched on- the current distribution along the length of the vertical radiator.
.
A proper HF mobile vertical is a combination of elements: paying attention to the ground losses, the Q of the loading coils, the diameter of the radiator- And-- The position of the loading coil.
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Determining the optimum position of a loading coil is beyond what I am about to get into here. It is sufficient to say that moving the coil above the base will result in a greater current flow along the radiator, and thus increase its efficiency. How much of an improvement?
.
This morning I ran a short program to give an example of what might be expected of a 40 metre antenna ( at 7200 KHz)--
.
Using a hypothetical 10 foot radiator with an assumed ground loss of 2 Ohms, the radiation efficiency for a base mounted loading coil with a Q of 400 (typical) is ~40%. Not bad. If the same coil arrangement is placed about 5-6 feet above the base (I won't get too specific here as to why- this is, after all, just a computer hypothetical model) the efficiency goes to ~55%. On the other hand, if it is moved towards the top of the radiator this starts to drop off-- drops off a lot,-- so by the time it reaches the top it is down into the 30's.
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.....Favors, definitely the centre loading coil design, No ?.... Its all about current flow.
.
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An interesting side note here, and one that goes back to my old friend, Ground Loss- If I increase that Rg to 10 Ohms, the same vertical's efficiency drops to 18% at the base- going only to a maximum of 35 with the coil at the approximate centre.
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Remember, that Ground Losses here are the antenna's base heigth-above-the- earth dependent. You have *some* control over that with a mobile station.
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Want another interesting variable?-- Look at the Q of the loading coils. I won't run that set of numbers, but it is shocking to see the difference between, say, a Q of 100 compared to one of 400.
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Okay, at 7MHz its approximately twice the loss-- Point?... use high Q coils.
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_______________________________________________
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I, personally, don't like or use any centre coil loaded mobile antennas. They are mechnically awkward to maintain and esthetically 'disturbing' to me. That's last critique is just me. Our work vehicle HF radios are all auto tuned base load'd stainless steel whips ~ 6 feet in length. They do Ok from about 6 Mhz up.
.
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But if I were going to put an antenna on an Air Stream trailer-, I'd use a centre fed vertical, as high up as I could mount it, with a base mount'd auto tuner-- for all my above reasons.
.
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...............................CF
.
.

Last edited by Coyote-Frostbyte; 01-15-2018 at 1:04 PM..
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Old 01-16-2018, 1:21 PM
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I am going to throw one more little twist into this mix- and then SK on this subject..... Its the placement of the tuner in relation to the vehicle.
.
For all intents and purposes, be it an auto or manual- tuner should be mounted directly at the vertical's base.... connected by a piece of copper braid measuring only inches in length. For a fixed outside "base' antenna, this is no problem- but what about in a vehicle (trailers too) ?
.
99% will be mounted inside the body- and hopefully right at the base of the antenna..... "inches only of connecting braid." Any length of connector within the vehicle is a loss- and longer the worse- That's not to mention the RF being radiated inside the vehicle.
A few years ago I ran a test of the difference between an interior mounted auto tuner (short 3" strap connection) against an identically placed external tunner,-- same antenna, same 3" braid--
I did this at an antenna test range with all their good stuff.
.
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Results ? Rather shocking. At 8 Mhz; a 1 db difference less with the internal mount. Lengthen'd the braid to 18" and the loss jumped another db.
Why is this..? Well, the increased strap length is intuitive. But with the same short connector, the only difference being inside the vehicle?
Think what goes on in a coaxial feed-thru bypass condenser. You want that hole thru the vehicle body to be as big as you can be comfortable with-- and use the best quality base insulator. Anyone that has worked with low band whips knows this instinctively, but probably never thought about it.
I now vector in those figures whenever I am thinking HF mobile antennas.

.
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My work vehicle has this big clunker of an auto tuner mounted right on the pillar separating the driver's door and the side window. The antenna's base is about 4 feet above ground, high up on the side of my Blazer. Directly on the other side of this pillar is the tuner -- a 3" copper braid connection thru a Big!** hole in the first double wall, with a industrial composite base on the exterior. The antenna is a standard commerical stainless steel ball and spring, with a 6 foot whip. With that RF centre only a few feet from my body I very seldom ever go QRO.
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......................................CF
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.
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** The esthetics here leave a lot to desire. The radio shop guys were a little hesitant to cut a 2 1/2 inch hole through that interior wall, but they found a large rubbber grommet to give it a finished look.
.
"They are not going to be too happy with you, Lauri, when you turn this vehicle in"
.
But with all the other 'toys' installed and holes drilled, none of my passengers have ever comment'd about that big crater.
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  #27 (permalink)  
Old 01-16-2018, 1:50 PM
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CF - can't let you SK on this just yet. A question: Where is the loading coil on your work vehicle located?
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  #28 (permalink)  
Old 01-16-2018, 3:40 PM
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Smiles-
.
There isn't any...its all done by the auto tuner and its circuitry. A 6 foot whip is not much of an antenna, but it will tune down to 6 MHz - at that frequency the power has to be dial'd way back or things start to arc-- but we never use anything that low.
8-10 mhz is the usual range, All things consider'd, its about 8-12% efficient there-- considerably more as the frequencies increase.
.

.
.
.....................CF

Last edited by Coyote-Frostbyte; 01-16-2018 at 3:46 PM..
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  #29 (permalink)  
Old 01-17-2018, 8:34 AM
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The best way is put the transmitter in RTTY mode and reduce power.
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Old 01-17-2018, 12:15 PM
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"tune
The best way is put the transmitter in RTTY mode and reduce power."
.
.
I'm not quite sure if this refers to anything I have written, but I do agree, UQA, that it is always a good idea to tune in low power.... an unmodulated continuous carrier is what auto tuners prefer (hence, I guess, your RTTY setting.) And if you are tuning them at their extremes- ie: trying to match a low frequency with a short antenna - they tend to arc and spark in defiance if done in HP....

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__________________________________________________ __________________
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I will add, that in my HF mobile case, above- that the use of just an internally mounted auto tuner is a big compromise. Yes, those clever devices can match the crummiest antennas- (maybe for my next act I'll see what they can do with a piece of brine soak'd string... just kidding..) but they exact a heavy toll. This becomes, in essence, an internally mounted base loading coil- and of marginal Q at that... hence contributing to the poor efficiency figures of 12% at 8 Mhz.
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There are some niffy commercial epoxy inline coils that could be added externally- any 2-way radio techs recall the Motorola coils used to put a low band whip down on 25 Mhz?-- something like these are what I mean. But we are not working world wide DX- only NVIS stuff over mountain ranges of just a few hundred miles. ...... and I have this 'esthetic's" issue....
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......................CF
.
.

Last edited by Coyote-Frostbyte; 01-17-2018 at 12:33 PM..
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