Mobile 20m 1/4 wave

[AD7N]

EDIT: Note, I'll be keeping this below 10-12ft, Please read entire thread before posting, thanks!.

I am building a mobile 20m 1/4 vertical whip (yes, 17ft tall). It'll be stiff enough to stay up at least 45 degrees at freeway speeds, and yet be flexible enough to bend below overpasses, signs, etc.

If there's powerline's overhead I'd take it down beforehand, so safety there I am watching for.

I've seen monster CB antennas on the road, so I'm sure whips that tall can be made. However, I'd like to get you're practical experience with this if you have any.

[WA9CWX]

Not sure, but my GUESS is it may be not be legal.

I am not a commercial driver, but I think max height is 13 feet.

Now, out west, where there may be 2 bridges in 500 miles, it may not make any difference, but around here, you would be hitting SOMETHING every few hundred feet...

More importantly, I DOUBT there is much difference between a LOADED whip of say, 9 or 10 feet, and a full quarter wave.

I am SURE there is a difference, BUT I am not sure the extra hassel of 17 feet is WORTH the difference.

I personally would spend more time and energy making sure the GROUNDING was complete, like every bit of sheet metal, etc. bonded.....and that the match was decent, AT the antenna.

As for the actual antenna, a helical whip can be very efficient, but the bandwidth will be limited. A base loaded would be the least efficient I believe, but would be the easiest to 'tune up'.

I still would NOT consider a 17 foot antenna while driving, BUT if you do, I sure hope to work you, and good luck with the project !!

Frank

[K9STH]

Back in the 1960s there was a California amateur who had a full sized quarter-wave for 40 meters on the back of his Volkswagen Beetle. He had it motorized so that he could lower the antenna for underpasses. He definitely had a VERY good 40 meter signal. Whenever he was transmitting and was in a situation where he had to lower the antenna he just said "QRX a moment" and lowered the antenna.

There were some photos and a brief article in QST about this mobile antenna.

Now I don't know about various state laws concerning the height. I do know that the laws are written concerning the height of trucks and trailers but not how they affect things like antennas.

Glen, K9STH

[AD7N]

Just reading into the legal code, it looks like WA state requires everything on a vehicle to be under 14ft (above ground), and 13.5ft is a common figure floating around the internet for most states.

However, I think you're right about the whip height. A good, sound loading coil and a well-connected ground would do more than 3-4 ft extra height.

Also, in doing a mobile vertical, I just want to make sure I have one thing correct: do you wire the braid of the coax to the body?

[AI6DX]

Way too high. You're gonna hit something, the top portion is going to break off and it'll fly right into someone else's vehicle. Not cool.

Do yourself a favor, get a bug catcher of some type. Some type of coiled vertical (Hi-Q, Hamstick), screwdriver or whatever. Don't even think about a 17' antenna that is not designed for automotive use.

It's ridiculous and if your antenna caused damage to someone else, you bet you'd probably hear from them at the accident scene!

[AD7N]

Tying the whip down would give me the same length, but would keep it secure and at a lower height.

I've seen whips tied down on some vehicles. How would a long, tied down whip compare to a bug catcher/screwdriver for NVIS activity? (This is assuming proper grounding etc in both cases)

[K7FE]

EDIT: Note, I'll be keeping this below 10-12ft, Please read entire thread before posting, thanks!.

I am building a mobile 20m 1/4 vertical whip (yes, 17ft tall). It'll be stiff enough to stay up at least 45 degrees at freeway speeds, and yet be flexible enough to bend below overpasses, signs, etc.

If there's powerline's overhead I'd take it down beforehand, so safety there I am watching for.

I've seen monster CB antennas on the road, so I'm sure whips that tall can be made. However, I'd like to get you're practical experience with this if you have any.

If a "no loading coil" antenna is your goal, I would suggest that you do the following:
1. Limit your maximum antenna height to 13' 6" from the ground to the top on the anti corona ball.
Tall, 1/4 wave HF antennas on a vehicle are every mobile operators dream. That is best accomplished while parked. The hazards presented by low power lines, overpasses, trees and other objects caused the states to enact legislation to limit maximum vehicle height and they vary a bit by state. New England and some others areas have some very old overpasses that did not allow as much vertical clearance as is now required by some 18 wheelers.

I limit my antenna maximum height to 13' 6". In my case that means that my antenna length can be 12' 6", since my mount is located behind the drivers rear wheel well and bolted to the side of my trailer hitch support at about 12 inches from the ground. The Highway Patrol has stopped me and measured my antenna height. Fortunately, I did not exceed the height limit.

2. Add a capacity hat as high as possible on your antenna.
The capacity hat addition will lower the resonate frequency of your antenna by adding capacitance at the top and tune out the reactance caused by a radiator whose length is less than a 1/4 wave (90deg). A large enough capacity hat will cause your 12' 6" antenna will act like a resonate 1/4 wave whip on 20 meters and no loading coil will be required. The efficiency will be about 75% of a full size 1/4 wave radiator on 20M.

3. Bond your vehicle metal parts to each other.......very well.
The metal parts of your vehicle ARE the other half of your antenna and contribute greatly on how well you radiate a signal and your antenna base impedance match.


4. Guy the antenna so that it does not tilt back at freeway speeds.
The tilt detunes the antenna because on one side it is closer to earth and on the other it is further away from the vehicle body which forms the other side of the antenna. Remember, you tuned it while the whip was vertical and you were not moving. Additionally, the antenna becomes more directional with a tilt and the take off angle changes. As it tilts, it becomes more NVIS..... as in radiating straight up, not out.

Other options:
A "good" center loading coil will work almost as well and is less likely to make contact with trees, so your mobile station will be more reliable. I make my own loading coils for bug catcher antennas and it is easy enough for most amateurs to do the same.

Let us know what you do.
73,
Terry, K7FE

[KA9VQF]

Here is a link to a PDF file that may interest you.

http://www.arrl.org/tis/info/pdf/0004033.pdf

[KC4YLV]

Have you thought about trying an inverted L setup? Mount an 8.5 foot CB antenna on the front bumper, have a horizontal element from the top of the antenna going back (securely attached and insulated, of course).

It's not going to have an omni pattern, but you will have decent efficiency and will have some vertical and some horizontal radiation - possibly an advantage.

Plus, you could experiment with putting loading coils between the whip end and the horizontal element for lower bands....lower efficiency, but something fun to mess with :)

[WA9CWX]

As for tieing it down and using it as an NVIS antenna, THAT is what the military does, and it is VERY effective for short range comms, ON NVIS frequencies, which are NOT 20 meters...!

The usual frequency will be under 5 mhz. It would never be on 14 Mhz, and on 14 Mhz, a dipole at six to ten feet off the ground will WORK, but NOT in any reasonable manner for the usual purpose of operating 20 meters !

Get a normal whip, bond any unbonded sheet metal if you want, and vote me in also for guying the antenna.
THAT is what I do. I have some rather thin nylon rope, and it is connected to the luggage rack, and slips over the HF whip, when I put the antenna on, otherwise, it is just tied onto one end of the rack.

[VK3FTIM]

EDIT: Note, I'll be keeping this below 10-12ft, Please read entire thread before posting, thanks!.

I am building a mobile 20m 1/4 vertical whip (yes, 17ft tall). It'll be stiff enough to stay up at least 45 degrees at freeway speeds, and yet be flexible enough to bend below overpasses, signs, etc.

If there's powerline's overhead I'd take it down beforehand, so safety there I am watching for.

I've seen monster CB antennas on the road, so I'm sure whips that tall can be made. However, I'd like to get you're practical experience with this if you have any.

Here's the upside for you it will work on 10 meters as a Half Wave :)

[K9STH]

FTIM:

Not really! A half-wave vertical will have a VERY high feed point impedance and will require a substantial matching network to use with any "normal" feedline whereas a quarter-wave vertical will come much closer to matching 50 ohm coaxial cable directly.

Glen, K9STH

[AD7N]

As for tieing it down and using it as an NVIS antenna, THAT is what the military does, and it is VERY effective for short range comms, ON NVIS frequencies, which are NOT 20 meters...!

The usual frequency will be under 5 mhz. It would never be on 14 Mhz, and on 14 Mhz, a dipole at six to ten feet off the ground will WORK, but NOT in any reasonable manner for the usual purpose of operating 20 meters !

Get a normal whip, bond any unbonded sheet metal if you want, and vote me in also for guying the antenna.
THAT is what I do. I have some rather thin nylon rope, and it is connected to the luggage rack, and slips over the HF whip, when I put the antenna on, otherwise, it is just tied onto one end of the rack.


Thank you for that note, CWX: I do like to work DX, but mobile that's not really my expectation. I'm more looking for "extended local range", and what you're describing is PERFECT!

Here in Washington State, I'm going to be driving around the mountains, and repeater/VHF contacts are near impossible more than a dozen miles away. I'd like to talk 1-200 miles, and having a stable NVIS channel of communications is a huge plus.

So, say something like this: (pardon the poor sketch)
http://i38.tinypic.com/2njyikl.gif

It would probably give me 25+ft of wire to use (far end being insulated from the body), and with a properly bonded car body and properly tuned I would think NVIS at 100-200 miles would be available most of the time. From what you're saying 80m is best... what about 160 and 40m?

[KI4UTI]

I think 160m will be about useless mobile, the ERP will be extremely low, ant S/N from such or short antenna would leave reception almost def. 80m and 40m is what you should be looking at. 80m is a challenge mobile but can be done, and 40m is relatively much more achievable. I recently got an 80m homebrew mobile antenna up and running this weekend and when I get it photographed and posted on a another forum I will try to remember to come back here and post a link. It got me a 54 report daytime (from VA) into NY and tonight I used it to get into an 80m net with 59+20dB reports into KY and WV and a 59+10 into NY.

[K9STH]

UTI:

There used to be quite a few mobiles on the 160 meter band running 10 watts or less on AM that "got out" for several hundred miles! Also, the old "police band" was between 1600 kHz and 1800 kHz and those mobiles worked over all of several states (especially those in the northeast). The old MF bands worked much better in mountains than VHF or UHF could ever hope to cover.

Glen, K9STH

[AD7N]

I am still curious what makes 160m/80m/40m bands better for NVIS than 20m. Is it band propagation? takeoff angle...?

[KI4UTI]

It's the MUF.

[W7LPN]

I run a 102" stainless whip on a 4' mast, with a coupler under the edge of the truck bed. I bent the whip at a 45 degree angle about 4 feet from the tip. It never hits bridges or other fixed structures, just a low tree limb once in a while. The bent allows it to deflect without bending. The bend doesn't appear to hurt performance at all.

[K9STH]

UTI:

MUF is defined as "maximum usable frequency" and really does not apply as to how NVIS works on 160, 80/75, and 40 meters.

Now one thing that does help with the 160 meter band is the fact that there IS groundwave propagation on that band whereas on any of the higher frequency amateur radio bands groundwave does not exist. That definitely helps with the "close in" work.


HQY:

It is basically that for NVIS the antenna has to be near the Earth's surface in terms of wavelength to achieve the very high angle of radiation required. On the 160 meter and 80/75 meter band this is very easy to obtain and fairly easy to obtain on 40 meters. But, by the time you get to the 20 meter band the antenna is going to have to be almost on the Earth's surface. On 20 meters a quarter-wave is less than 18 feet and then you have to consider that in most soil types actual r.f. ground is between 6 feet and 8 feet below the surface. That means a 20 meter antenna 10 feet above the Earth's surface is a quarter-wave above r.f. ground and that is a reasonable height to start getting a decent angle of radiation. To get the very high angle of radiation from a simple antenna that means the antenna is going to be only a foot, or two, above the Earth's surface.

Now if you were to point a yagi straight up then you will get the high angle of radiation necessary for NVIS. But, how many amateur radio operators point their yagis "straight up"?

Glen, K9STH

[KI4UTI]

For NVIS the atmosphere has to be able to support the radio wave going up being reflected straight back down at the frequency used otherwise the signal escapes earth. (N.V.I. near vertical incidence ;) )

A little primer on NVIS, google has tons more.
http://en.wikipedia.org/wiki/Near_Vertical_Incidence_Skywave

An NVIS mobile installation.
http://www.tactical-link.com/nvis3.htm

[KI4YTV]

how about 2, 10-11 meter whips mounted in a v configuration ?

[VE7NOT]

In the mid 90's when everyone was into AM CB I saw a camper with a 'bigstick shakespeare' (17') on the back bumper..... (nowadays everyone runs 102" whips and uses ssb)

Since then I have looked up websites with 2200m mobile antennas... 20m is nothing...

I once causally broke into a three way qso between a ham in alaska, a ham in arkansas, a ham in houston tx and was told by them that my mobile station (a 20m hamstick with a 706MKIIG) was 5/9.

The Houston ham said I was s-9 and th the others had me at s-4 to s-5 or so....


Later that day I signed onto QRZ and got a PM from a ham on the east coast of the US (PA or NJ I think..) who said heard me....

So a 7' hamstick on 20m and 100watts from a 'simple radio' (by today's standards) was enough....

A 17 footer will be a good 1/4 wave... but put simply unless you live in area free of trees and are ready to beat up teens who make fun of your car... try a simple antenna first....

[N0EQ]

...I have looked up websites with 2200m mobile antennas...



That's ONE long antenna!
Hefty vehicle mount, to be sure!


Craig 'Lumpy' Lemke
--
Camo Battery Powered Desert Ops
www.n0eq.com (http://www.n0eq.com)

[W7XF]

As far as antenna height....

If you are WEST of the Mississippi River, the statutory height limit for any vehicle is 14'0"
If you are EAST of the Mississippi River, it is 13'6".

Take this into mind when building mobile antennas.

However, MOST newer overpasses on the Interstate system are at 16' vertical clearance minimum.

(I am a commerical driver...this is gospel)

[AD7N]

However, MOST newer overpasses on the Interstate system are at 16' vertical clearance minimum.

13.5ft is more than enough to do a bent-over antenna for NVIS. However, a full vertical would be nice, but alas those low-hanging powerlines will always give you a hard time.

;)

[KC1HD]

Whatever design you choose, do yourself a favor and visit Alan's website on mobiling at www.k0bg.com

[W7LPN]

I've done this for a few years now. If you use a cap hat 3/4 way up it will improve radiation at the horizon. I ran several configurations and on my Dodge P/U this type worked the best. I've used auto tuners/autocouplers/impedence matching box, all with good results and the autocoupler & cap hat slightly above the rest.

[M3KXZ]

I've done this for a few years now. If you use a cap hat 3/4 way up it will improve radiation at the horizon. I ran several configurations and on my Dodge P/U this type worked the best. I've used auto tuners/autocouplers/impedence matching box, all with good results and the autocoupler & cap hat slightly above the rest.


Problem with the cap hat 3/4 of the way up is that it cuts off the top 1/4 of the antenna which would otherwise also contribute to raising the current max higher up the antenna. Unless the cap hat is massive and over compensates for cutting off the top few feet of antenna then it seems sort of awkward. Placing the cap hat right at the top would be better, or placing the cap hat at the 3/4 mark and being able to benefit from chopping the top quarter off, to be sure...

Pete

[M3KXZ]

UTI:

MUF is defined as "maximum usable frequency" and really does not apply as to how NVIS works on 160, 80/75, and 40 meters.

Now one thing that does help with the 160 meter band is the fact that there IS groundwave propagation on that band whereas on any of the higher frequency amateur radio bands groundwave does not exist. That definitely helps with the "close in" work.


HQY:

It is basically that for NVIS the antenna has to be near the Earth's surface in terms of wavelength to achieve the very high angle of radiation required. On the 160 meter and 80/75 meter band this is very easy to obtain and fairly easy to obtain on 40 meters. But, by the time you get to the 20 meter band the antenna is going to have to be almost on the Earth's surface. On 20 meters a quarter-wave is less than 18 feet and then you have to consider that in most soil types actual r.f. ground is between 6 feet and 8 feet below the surface. That means a 20 meter antenna 10 feet above the Earth's surface is a quarter-wave above r.f. ground and that is a reasonable height to start getting a decent angle of radiation. To get the very high angle of radiation from a simple antenna that means the antenna is going to be only a foot, or two, above the Earth's surface.

Now if you were to point a yagi straight up then you will get the high angle of radiation necessary for NVIS. But, how many amateur radio operators point their yagis "straight up"?

Glen, K9STH


I know this is an old thread....but I think there is some misinformation here. Glen, what makes you think MUF doesn't apply to NVIS communications on 160, 80 and 40? The MUF is VITAL to how NVIS communications work on these bands - MUF is the critical frequency above which radio waves are not reflected by the F2 layer when the angle of incidence is vertical. If the MUF is lower than the frequency you are using, then NVIS communications will not work, as the RF will go straight through the F2 layer and into space rather than being reflected back down to earth. And as for 20m, well unless you live somewhere where there is often an MUF greater than 14MHz, then that's a complete no-no. I don't know quite why anyone would want a high angle of radiation at 14 MHz.

Now when looking at longer paths, where the angle of incidence is nowhere near vertical, then the figure to look at if the MUF(D), for whatever length of hop you're looking at - i.e. the map might show an MUF of only 5 MHz at a point halfway along the path, but you could well have a MUF(D) at that point for a 1500km hop (for example) - known as MUF(1500) - of 14MHz.

[M3KXZ]

I am still curious what makes 160m/80m/40m bands better for NVIS than 20m. Is it band propagation? takeoff angle...?
As UTI said - it's the MUF. Have a look at the ionospheric maps here http://www.ips.gov.au/HF_Systems/6/5 . Look at where you're location is, and see what the MUF (maximum usable frequency) is. You need to be operating below the MUF for your very high elevation radio signals to be reflected back down from the F2 layer. The higher the frequency you can use, while still being below the MUF, the better as the D layer absorption is lower.

The good thing about running an antenna for 160, 80 or 40 fairly near to the ground for local comms is that the antenna ends up pretty deaf to signals from further away, and you get a much better signal to noise ratio for the closer in signals. It works a treat and is the reason why local (out to a few hundred miles) ragchews are so easy on the lower bands.

Pete

[W7LPN]

Problem with the cap hat 3/4 of the way up is that it cuts off the top 1/4 of the antenna which would otherwise also contribute to raising the current max higher up the antenna. Unless the cap hat is massive and over compensates for cutting off the top few feet of antenna then it seems sort of awkward. Placing the cap hat right at the top would be better, or placing the cap hat at the 3/4 mark and being able to benefit from chopping the top quarter off, to be sure...

Pete
K0BG says that this is a common misconception. Above the cap hat the RF changes directions, and not straight up either, and fools some folks. Also, it improves reception. I didn't have any expensive test equipment, but tried it with & without the whip above the cap hat. The performance was far better with it. I'm not sure I can, or even want to, argue as to why, but in my "Real World" experiment, it was. I kept Alan's e-mail...maybe I can find his explanation. I know this also from when I ran the bug catcher with a whip above the cap hat. Anyway...just my experience and learning. :cool:

[KI5DQ]

Please keep the overall height (ground to antenna tip) <13.5 feet.
This is a requirement in many states and keep you from receiving a citation from a police officer.


de,
KI5DQ

[F4FYJ]

Hi,

Look at mine :D
It's a half rotative 10,15,20 m dipole wich was broken last year by a strom recyled in a vetical mobile antenna.

3,7 meter high .
I can't divre with this antenna in france .
http://img.over-blog.com/600x450/2/67/67/50/IMG_2234.jpg

[K1DNR]

FWIW - I said this in another thread. On a car trip this summer, I did 20 meters HF with a 4' CB mag mount antenna (fire stick), 12' of RG58 and an MFJ antenna tuner. There was no supernatural propagation and I worked a bunch of stations, including Europe several times, from the road driving through Ohio and Indiana on I80.

Certainly a hamstick that was designed for 20 meters would work a lot better than that.

I'm only posting to give you a "crappy antenna performance baseline" - and my crappy 4' CB antenna did an ok job.

Not sure you need to go to all this trouble.

Hamsticks are cheap. Reasonably easy to mount, etc...

[WA7PRC]

I've run modified Hustler/New-Tronics (http://www.new-tronics.com/) stuff, starting with the MO-1 mast:

75m = RM75 w/ inductance removed + 56" whip (129" OAL)
40m = RM40 w/ inductance removed + 40" whip (111" OAL)
20m = RM15 + 39" whip (108" OAL)
15m = RM12 + 25.5" whip (91" OAL)
10m = NO resonator... MO-1 mast + 40" whip & homebrew adapter*

The 20m setup has worked well enough to work into Europe (on CW). I s'pose one could use a longer whip with less loading coil. I haven't tried that... yet.

All whip lengths are only the part that protrudes above the resonator. You'll need a whip at least a few inches longer than that. The RM75 uses the Larsen W640 (64") whip. Others are either Larsen W490 (49") or Hustler/New-Tronics.

* The adapter is made from a 3/8"-24 coupling nut + 1/4"-28 socket head screw - see attached drawing. The (old but still available) Hustler locknut + split ferrule is used with it. A 1/8" hole is drilled through the stud at the top of the mast tubing.

Photo of 10m adapter:
http://farm5.static.flickr.com/4118/4903254975_cfc59fdfee_m.jpg