AM Mobile - Tarheel Model 75A ?

[W1TAV]

Over the the years I have run different mobile antennas, (Hamstick's, Hustler, Homebrew) with varying results. Now that I have gotten that elusive "round tuit"  and installed a 857D in my Pick-up, I am looking at the Tarheel Model 75A. I understand that any mobile installation is a compromise. In this case, I need to keep things in the bed of the truck. Mounting it off to the side or on the hitch is not an option due to regular (work related) off road use.  I will be using it with a capacitance hat.

I am primarily on 3.885 and 7.295.. With the improving solar conditions, likely 29.000 too..

Thoughts, experiences, advice welcomed..

Steve  

[KB2WIG]

Any chance installing it into the  'stake rack' hole?? 

What ever u du, make sure you get a real gud ground fer the ant base.

klc

[WB2EMS]

I think you would do better with a longer version like the 200 or 400. I ran for several years with a BB3 screwdriver antenna that was about 4 foot tall, plus whip, 2 inch diameter. For the low bands, I ran a full length (102") CB whip on top. It was mounted higher than trailer hitch, actually the base was even with the bottom of the window on the rear of the van, which helps with ground losses.

I had many good QSO's on 75 and 40 meters in that configuration running an Alinco DX-70 with 25 watt carrier. It would tune up through 20 meters with the long whip on, maybe 17. To get above that, I had to change out the whip to a 60 inch whip, and then it would tune up through 10 meters.

If your focus is mostly 20 meters and up, then that stubby version of the screwdriver will likely be adequate. For 40 it will likely do ok, certainly better than a hamstick, but not be as good as it could be. On 75 I think you will be giving up a bit with the short length. IMHO. YMMV

[n1ps]

Hi Steve,

I concur with the above points.  Antenna base ground is critical and you want the longest antenna you can get away with.  On the former point, I designed a steel bracket for my center loaded antenna....and threaded it into the frame of the car.

As you know I am frequently 80M mobile.  I happened to make a homebrew antenna with a 3" coil.  I also use a base matching coil to match the low Z of the antenna to 50Z.  In this case I used the Texas bug catcher matching coil, a simple and elegant solution.

As far as commercial antennas, the screwdriver types are the best. Use the tallest antenna with the largest coil you can get away with.  From what I have studied and seen at the festers, I recommend the Hi-Q antennas. I like their design in that the coil does not move up and down (IIRC).  But they all are pretty good.  Avoid the short ones like noted above...you may as well run a hamshtick

If you run a diesel engine in that truck you gain a lot in receive noise.  The only engine noice I hear are the vehicles around me...

A linear is a good idea too.  Makes us PW mobiles a little less so. 

I love my HF mobile.  Can't beat that 75M entertainment.  Hope to hear you out there...join the p!ssweak mobile club

Pete

[AJ1G]

Speaking of piss weak mobile, I have been having a lot of fun of late with an Elecraft K1 that was loaned to me by buddy Steve, KL7JT.  5 watts of CW goes a long way on 40 and 20 using my roof rack mounted Hamsticks, all the way to Europe, Ecuador, and all over the USA!  I've had great luck over the  last 10 years with my FT100D at 20 times that power, but am really amazed by the little K1.  And boy its RX hot (and quiet) in it!

[Fred k2dx]

If you change bands often it's tough to beat the SGC system for all band convenience. The Helically loaded whip is efficient, though on 75m not quite as efficient as a long whip/big coil but reasonable. On the other bands there isn't a difference that I've seen. I have been using the SG230 tuner/SG303 antenna for a long time and don't miss the Hustlers w/kw resonators at all. The SGC automatic QSY with RF sensing is convenient. Icom makes a knockoff tuner. For best results the tuner must be as close as possible to the whip base.

As has been said, the ground is all important for mobile performance. A pickup bed should be very good. I bonded doors, tailgate, exhaust system, frame, hood, body together with wide braided straps on my Blazer. I've had QSOs with all continents. I use a second Optima battery next to the tx with an isolator for minimum voltage drop and noise reduction.

[K1JJ]

As has been said, the ground is all important for mobile performance. A pickup bed should be very good.

Yes!!

Back in the mid-90's a bunch of us ran exhaustive 75M mobile tests. We built our own monster mobile antennas, coils - used 1KW linears, etc.

The bottom line is that the limiting factor is the ground losses. We can have the best vertical, even a full size 60' whip, but the ground losses of a small vehicle groundplane are always the overriding limiting factor to signal strength.

The point of dimimishing returns appears to be at the "Bug Catcher" level. Whips shorter and more lossy, like Ham Sticks, are down maybe 6db from a BC.  But there appears to be no way to improve much on a BC or even a Screwdriver.  I like the Screwdriver cuz it's remote tunable and is close to the BC ant in performance. 

All you can do is add ground radials to increase efficiency on a mobile.  We even tried large copper tubing coils wound on 5 gallons plastic pails and whips that were 20' high made from copper pipe. The ground losses just eat it up.

The best mobile installation is like what Warren, K2ORS did. Attach some small 60' wires to your car frame and tow them down the highway. That actually works until a car runs them over...

I once did an A/B between my truck BugCatcher and a dipole at 60' here in the backyard. The BC was always about -15db down from the dipole, no matter what I did to the truck.  That's about the best we can do mobile unless we go fixed mobile and run out some ground radials.


The best time to run tests appears to be about 1-2 hours before sunset when the verticals work reasonably well locally.
T

[WB2EMS]

The gold standard seems to be the bug catcher types, especially with a properly located capacitive top hat, but a decent sized screwdriver doesn't give up much against it. Hamsticks are down more than 6 db and the wander lead helical wound antennas are dead last. At least according to the various shoot outs I've seen.

In my best installation, I had a shelf just at the bottom of the rear van window with good 2 inch wide copper bonding straps from it to the rear hatch, and from the hatch across the hinges to the body of the van. Lots of sheet metal all bonded together in the van body gave a fairly good ground, and getting the base of the screwdriver up away from the road helped with the ground losses. The arrangement allowed the rear hatch (dodge minivan) to open normally with no impediment. Looked odd, but worked well. 

I used a matching coil of several turns at the base with two taps. Full coil (4t, 2 inch) gave a good match on 75 meters, about half that worked best on 40, and back to full coil on 20 and up where it acts more like an RF choke and drops out of circuit except to provide DC ground. Using the coil helped a lot with rain static. Eventually I'd like to put a relay out to switch the tap on the matching coil.

In terms of grounding being the limiting factor, I've wondered for a couple of years about small magnetic transmitting loops as a possible way around those limitations. There is a mobile system that was original marketed by Q-mac of australia, and now apparently by Stealth which is a vertical loop on the roof tuned with a vacuum variable which claims a 10-14 db increase over a loaded whip antenna on the lower frequencies. Some day I want to rough out one on my vehicle for some static field strength tests and see how it looks. 

www.worldsecurity-index.com/.../qmac/MX9027%20ML-90%20HF-90M%20NVIS%20Vehicle%20Package_Nov04_web.pdf

http://www.stealth.ae/plugins/custompages/detinf.php?id=372&id_categories=136

Mobile HF is fun. Time to tame the noise in the new vehicle and get my gear reinstalled in there.

[W2PFY]

I'm thinking of running this coil on 160-40. I think it will tap down ok. I really haven't tried it yet. It's too large for the roof and it's quite heavy. The driven element would be too tall as well. Might hit a bridge!

[WB2EMS]

The best mobile installation is like what Warren, K2ORS did. Attach some small 60' wires to your car frame and tow them down the highway. That actually works until a car runs them over... Shocked

I thought about trying that, but more likely when bicycle mobile which has even less ground plane.

One that I do want to try is to take a wire from the top of the whip long enough to just miss touching the ground with a conical drogue on it, like what is used as a trailing wire from airplanes. The idea would be that under any reasonable speed of motion, the wire would stream back from the tip of the antenna, hopefully well above any following traffic. (might be a good discouragement for tailgaters) It would double the length of the radiating element. Of course it would detune badly when you came to a stop, but for those miles on the highways, might help on 75 and 160.  

[K1JJ]

 

Yep, just like a real Zepp.

Bear in mind, the ground losses of a standard vehicle on 75M are so bad that even a perfect coil and 60' whip would still be down around the level of the Bugcatcher. We tried it here.

It's like having a silver plated full size 60' vertical with no radials on the ground. No matter what you do to the top, the power is all wasted in the bottom half.

As the frequency goes higher, the vehicle coupling is better. That's why we see some great mobile signals on 40M and on 20M up. A mobile can approach a home vertical signal on 10-20M. On 10M we now have a full size vertical whip and the vehicle coupling to ground can be superb.

That said, when Warren/K2ORS works stations regularly on his 160M AM mobile, that's quite an accomplishment...

T

[W1TAV]

Thanks to all.. Tom Great Info from the old days..  Well I have broken out the 1" braid and embarked on a grounding everything project.  Bottom Line I have upgraded my plan and will go for one of the bigger "screw driver" type antennas. I have the real estate on the truck to mount it.. I'll just have to be creative and diligent in keeping it out of harms way when in the woods..  Bigger is better... 

Steve - W1TAV

[WU2D]

My 75 Meter micro Hustler Mag Mount is the tops for inefficiency and with 20 Watts of glorious AM, that is a tough combo. But some people do hear me. It can only get better from this thing.

Mike WU2D

[AJ1G]

Mike  - your photo illustrates exactly what I dislike about the Hustler center loaded coil - they act like a big pendulum and cause the antenna to whip around a LOT.   Everyone ends up having to add those  guy strings.  Adding a spring at the base just makes the thing move more.  I have a bunch of Hustler antennas and sometimes use them while stationary.  A to B comparison of them with the Hamsticks  in QSOs show no significant performance advantage of the Hustlers over the sticks.  Even if the sticks were 3 dB down vs a Hustler, I'd have to say that the sticks are at least 6 dB less Hamsexy.  So there's a net 3 dB gain for the stick!

Even it the sticks are 6 dB down from a big Texas Bugcatcher, that's only an S unit, and after adjusting for at least an 8 dB Bugcatcher Hamsexy factor, the sticks have a 2 dB advantage over them as well!

Tom Vu is right - what really is limiting are the ground losses.  That's one of the reason I now mount the Volvo sticks on the roof rack..the big roof as high as I can get off the road is about as much acreage you can get on a passenger car.  I remember reading somewhere that the higher the mobile antenna feed point is above the road the better in terms of minimizing ground loss.

[K1JJ]

Chris,

heheheh - That's funny - equating Hamsexy db to gain to normalize a mobile installation...

I guess what looks cool on a big tower in the backyard looks the opposite on a car.

Funny story: Back in 1994 I had a big homebrew Bugcatcher on my Blazer with spoked radials above the big coil. It looked ridiculous. One day I drove too fast into a 7-11 parking lot and cut off two rednecks in a pickup truck. They were POed and parked. I immediately got out and went over to talk to them as an aggressive pre-emptive tactic. I had just passed my Brown Belt, so felt bad-ass that day.  After a minute, the driver said I seemed like a good guy but that big antenna made me look gay!  HA!  He immediately floored the truck and burned out of the lot.

  I was morified. I took that huge HamSexy antenna off the same day and never put another one back on. It does attract a lot of unfavorable attention, I must say. Cheezz.


BTW, the mobile ground losses... to elaborate more: The car gets it's groundplane by using its body and frame as a huge capacitor plate. The other plate is the surface of the Earth. This surface may be dirt, concrete, rocks, etc. This huge capacitor has big losses due to materials involved and the low effective capacitance to ground thru the two "plates" for say, 75M.  It's like the antenna whip is in series with a big resistor. If the resistor "impedance" after everything is figured in is 1/2 the radiation resistance of the whip, then there is a 3db loss in the resistor/ground path. It is usually MUCH more than that, like a 15db loss on 75M.  If a whip is really poor (HamStick) then it will make matters even worse – even with a poor ground system. We start to fall into the abyss of PW.

The problem is that many of these 75M whip antennas may have a radiation resistance of 5-12  ohms or so. We need a super radial field to work with an antenna like this to keep the ratio of ground to whip values reasonable.      If a BugCatcher was used with a few radials over salt water, it would work superbly. That's why some guys will park their vehicles on the beach with salt water around the car for great performance.

Positioning of the whip is best in the center of the roof. This is too HamSexy, however. The mistake some guys make is to mount the whip on a lower side mount or bumper and a large part of the whip radiation is cancelled out (like feedline) by the close proximity to the vehicle body.  The body and whip are really two legs of a dipole and need to be run at right angles for the entire length of the whip, if possible. Mounting a whip at one end of the vehicle will also cause directivity, which is not a good thing when mobile, since we drive where the road goes...

[AJ1G]

Re directivity due to mounting an HF antenna on the end of the car vs centrally....I've "boxed the compass" is QSOs while in a parking lot, and have also monitored for changes in signal strength while turning and changing direction while underway...if there are any directivity related changes in signal level either transmitted or received, they are pretty small, at least at 40 and 20 meters.

Another thing that I've often noticed is that  when I am passing through areas where I really am in the hole, like between  steep and narrow rock cut cliffs on an interstate, HF signal strengths don't drop off that much, even when  the cliff is directly in the path to the other station.  Maybe there is some scattering/reflection going on?

Would be fun to have a mobile antenna shoot out sometime, maybe at fall NEARFEST?  They sure have enough open space at theat location to do one!

[K1JJ]

Hi Chris,

OK on the scattering. I guess that's all it could be if the signal didn't change when a big hill was in the way.  Unless high angle radiation was being supported best because of the valley situation. The vertical does not favor it but if that's the only path, then why not.

I hear Tron sometimes going over a metal bridge. He always says to watch how his signal comes up. It usually does due to the RF coupling and re-radiation.

Back in the mid 90's, Dino, Richaroni and I did some tests in the large Kay-Mart parking lot near Rich's QTH about 2 miles away. One of us stayed at his house watching the s meter while the others keyed their rigs completely in the clear. We did find some directivity on 75M when the vehicles turned positions. I don't remember the best orientation, however, but maybe it was with the whip facing the QTH, I dunno.

Dino once parked within 30' of the KayMart building, using it as a reflector. His signal came up about 6db. That was amazing. Or maybe it was coupling into the metal structure and re-radiating, I dunno.

We found there was very little difference between signals once a "BugCatcher" antenna was used as a minimum. Dino even added this monstrosity system using  3/8" copper coil tubing and a 5 gallon platic pail as the coil - and this very large whip supported by 2X4's in his truck bed. There was little difference between that and our bugcatchers showing that ground losses dominated. Though, Dino always did very well mobile on 75M.  

However, we did detect the difference between a BIG bed truck and a smaller Japanese car. The groundplane is important.

The ideal mobile vehicle would be an 18-wheeler with the ant on the trailor center roof... :-)

T

[AJ1G]

Here's the Volvo up at NEARFEST a few weeks ago - there's two antennas on there somewhere!

[K1JJ]

Very nice location for that Hamstick up on the car roof.  In the clear, at right angles and nice groundplane. That's the advantage of using the shortened helically wound whip.  I'll bet that works pretty well in the late afternoon on 75M.

A BugCatcher up there would smack the bridges and toll booths...  - but give a few few more db in effciency. With the BC, you might even make Hamsexy.com too.  It's always a compromise.

T

[WU2D]

Mobile trip - well I was barely heard going up on Thursday as we had really tough storms coming though Vermont. It turns out that I missed the tornado alert by about 1 hour around Barrie. I had already made it to Burling ton where I pulled into the Winooski Friendly's. Just as I got through the door and was already drenched from the 100' run from the car, as the restaurant the lights went out.  

Back on the road an hour and a Sundae later I crossed the Lake Champlain islands heading into New York. As I went over the Hog Island bridge (and with the 75M mobile off), I suddenly heard snapping inside the command transmitter that got worse. I thought that the HV supply had somehow come on and I had a short so I bent over to disconnect the power Jones plug and got zapped. The entire chassis of the ARC-5s were hot like an electric fence and snapping like mad.

This can be disconcerting at 45 MPH.

When I dipped back down to island level off the bridge on the other side it finally discharged enough that the snapping stopped.

The top of the bridge was at such a potential gradient that the Hustler was picking up a tremendous charge.

The moral of the story is GROUND YOUR ANTENNA either through a choke at the mobile transmitter's output or better yet use a grounded matching scheme at the feed in the form of a grounded tapped coil or a matching shunt L.

No damage in fact I had a long nice QSO with Timmy on the way back today.

Mike WU2D

[AJ1G]

Mike - re the zorching in the Command Set - WOW....whudathunk it?  I got zapped once on a Field Day really badly from a lead in of a very long wire vertical hung from a  balloon ...there were T storms in the area and Franklin's kite and key experiment was replicated....

Tom - re the roof mount sticks - the longest ones are for 80 CW and 17 meters where I use stingers about a foot longer than the stock ones so they resonate at the lower frequencies ( I use a 15M stick for 17).  On the Volvo all antenna heights are less than the 13" 6"" legal height  limit imposed on trucks, so I don't catch overhead wires, but I do bonk on low tree branches a lot - never have broken a stick yet though.  For the extended length stingers, I use the 1/8 OD stainless steel whip from Rat Shack CB base loaded antenna they used to sell, maybe they still do.  The mounting bracket is from a right angle mirror mount, using stainless steel hardware.  The Volvo racks are made of cast aluminum and are very rugged - I took them off to insert the mounting bolts from the inside out.  The coax shield is grounded to the roof just underneath the upper part of the rear cargo door lip out of sight, and the center conductor goes up to the antenna through a heavily taped over run under the door seal.  With the low Z feed point, have never had any arc over there at the 100 watt level. 

Just spent about an hour handing out the AJ1 multiplier in the CW WPX contest while parked down at Stonington Point.  With  5 watts from the K1 on 20, easily worked pretty much anyone I could hear.

[WU2D]

Chris,
Actually this would have not happened so easily with a stock command set as the output link is a short to ground through the roller. I had modified this set to a PI network output.

Mike

[AJ1G]

Was wondering about that - I assumed you were running the original link...thought maybe individual lightning strike pulses were ringing the tank circuit, and being stepped up by the turns ratio of the link to the coil....I've been lucky with the Hamsticks ---usually disconnect them when a storm is due though.

[WU2D]

Well my good pal W1VKZ Andrew (VK3CV) made me up a mount for the Maxima. In about an hour he cut and welded this nice little mount. It attaches to the tie down strap loop mounting holes which attached with 4 bolts. The Tie down weldment removed and mount installed, we are now ready to paint the thing. Now I have a foundation for a full sized whip on the rear of the car. The next step is to design the antenna!

Mike WU2D

[AJ1G]

Mike - suggest you put an insulated extension below the antenna so that the bottom of the antenna is no lower than the top of the trunk lid.  Ground your coax feed line just inside the trunk lid and run a short lead out to the base of the whip.  I have an arrangement like yours on my Escort sedan and it's performance with the feedpoint very close to the ground with a good portion of the antenna close to the sheet metal is notably off from down from the roof mount setup on the Volvo.  Smaller "footprint" of the Escort vs the Volvo probably also part of the difference.

BTW, in case you missed my posting in the Vertical Antenna thread, I worked ZL on 40CW with the 5W K1 one foggy morning last week from Stonington Point.