HF Mobile Install with Linear Actuator and Additional Groundplane
I collect topographical survey data for agriculture with my pickup. I start out by traveling around the perimeter of the field area, where there are often overhanging limbs that damage antennas. When surveying, I even have to replace the Larsen NMO-2/70B on top of the cab with a shorter VHF/UHF antenna.
I decided to use an electric linear actuator to lay an HF antenna down and stand it up from within the cab of the pickup. I made a T-bracket from C-channel to mount the hinged swivel and linear actuator.
Since my Toyota Tacoma pickup has a composite inner bed, I built a frame from angle iron to hold 18 gauge sheet metal at the top of the bed rails for additional groundplane directly at the feedpoint of the antenna. The sheet metal is also bolted to the top of the T-bracket and bonded with 4 ground straps to 4 of the 6 bolts that hold the composite inner bed to the frame. I had everything powder coated to make the sheet metal resemble a tonneau cover.
I was initially going to purchase either a Scorpion 680S or Hi-Q-6/80, but really wonder if either of them would hold up to the very rough terrain that I sometimes encounter. I have decided to run HamStick type antennas while actually driving through the fields, which I can inexpensively and easily replace. I drilled a 3/4 inch hole for the antenna mount, just in case I want to mount one of the motorized antennas or some form of Texas Bug Catcher. Anticipating a heavier antenna, I may build an even stronger hinged swivel and mount a cradle near the tailgate for the antenna to rest in when folded down.
I want to run 10 meters during the day when it is open, 17 meters during the day when 10 is not open, and 40 meters early in the morning and late in the afternoon when traveling to and from jobs. I am of the opinion that it makes about as much difference where and how a mobile HF antenna is mounted as it does the type of mobile HF antenna that is used, assuming the same physical length. I really doubt that any of the more expensive antennas would have significantly better performance on 10 or 17 meters, but I would like to have a heavy duty HF mobile antenna that is optimized for 40 meters that I will only use when traveling down the highway. Any suggestions? I have considered trying to come up with a heavy duty mast for one of the Bug Catcher coils and I have not ruled out purchasing one of the motorized antennas for this purpose.
Thanks in advance,
As I told you before, the Scorpion is rugged enough, but you have to secure it well. Any movement in the mounting assembly will increase the shock moment which is the killer in any mobile installation.
Alan Applegate, KØBG
This fold-over/additional groundplane is a prototype that I have always wanted to try. It is something that I threw together with C-channel and angle iron that I already had laying around the shop. The only things that I had to purchase were the linear actuator and the sheet metal.
The design evolved in such a way to allow me to easily modify the hinge/fold-over mechanism and then be able to bolt the additional groundplane back up to a beefed up version of the hinge/fold-over. Given the limited time that I had to work on this, I wanted to get my additional groundplane mounted first for test purposes. What I was trying to accomplish here was to have the primary groundplane directly at the antenna feedpoint, have the antenna mounted as close as practicable to the center of the metal of the pickup, have the antenna mounted as high as practicable above the lossy earth, and move the antenna back about 2 feet from the metal of the cab of the pickup.
Please note that I gravitate toward the higher bands (10 meters), when propagation permits, where my mounting configuration with the additional groundplane bonded to the rest of the metal of the pickup should result in a noticeable difference in performance. Most of the mobile antenna discussions I have read have concentrated on 80 meter mobile antenna setups. I have experienced where having the antenna mounted on the side of the vehicle on 10 meters would make as much as 2 S-units difference in the orientation of the vehicle relative to the other station, whereas there may have not been a noticeable difference if this was taking place on 80 meters.
I understand your caution of increase in shock moment of the antenna. Give me a little time and I will build a beefed-up version of the hinge/fold-over mechanism with a rubber shock mount that will cushion the end travel that results from slop in my fold-over mounting configuration. Have a little faith
The complete T-bracket and frame simply bolts to L-brackets bolted to the utility rack near the top inside of the bed rails. The whole thing can be fairly easily removed and whatever antenna can be temporarily mounted directly over the bed rail for comparison purposes. I am going to either purchase or build a Bug Catcher type antenna that is optimized for 40 meters (the lowest frequency band that I want to operate mobile) and use this antenna as a benchmark for test purposes.
For test purposes, I don't have any highly accurate test equipment and will therefore probably not be able to detect minute details. I am simply going to use an S-meter to see what the "real world detectable" differences are in a couple of mobile antenna types. For example, I want to see if I can actually detect a difference on the S-meter between a Hamstick type antenna, a Bug Catcher type antenna, and one of the top motorized type antennas on 40 meters.
For this type of simple testing, I can be self-sufficient. I have the WiFi interface installed in the radio box of my Remote Rig system. I can use my cellphone as a WiFi hotspot to supply internet to the Remote Rig box. One of the TS-480 control heads will be connected to the body of the TS-480 in the pickup, which is connected directly to whatever mobile antenna that is mounted on my pickup. The other TS-480 head will be connected to the Remote Rig box, to give me access to my internet remote base, from my pickup. I can either use one of my present, mostly horizontal antennas, for testing or connect a vertical antenna for test purposes. This test setup will also allow me to compare mobile antennas on both transmit and receive.
As a side note...One of the reasons that I purchased the WiFi interface for my Remote Rig Radio Box was to allow me to access my internet remote base from my pickup. I have found that internet remote base does not work well while in motion because of dropped connections when you switch cellular towers, etc. When stopped at a location with stable internet, I can pick up the microphone that controls transmitting through the mobile antenna on the pickup and get a signal report and then immediately pick up the microphone that controls my internet remote base and compare my mobile signal to my base antennas.
Last edited by NZ5E; 01-23-2013 at 04:45 PM.
Under nominal conditions, the difference between one antenna and another can be anything from a few tenths of a dB, to as much as 20 dB. The problem is, no matter what methodology one uses, the instantaneous changes in propagation can be even deeper than 20 dB! Obviously, this is beyond any reasonable S meter.
You can use the background hash on a dead band which tends to be rather stable over a short period of time, there are so many variables, even this method has its limitations.
The other variable which is hard to control is ground loss. It is important to remember, it is in series with the antenna's input impedance. The amount of current flowing through it, depends on the other losses making up the antenna's input impedance. In other words, the difference between one antenna versus another is easily masked by a dominance of ground losses. Remember, the loss is a square function of the current flow.
Alan Applegate, KØBG
yep those composite beds are evil, im still planning my HF antenna install, I am installing a scorpion SA-680 (someday soon), currently trying to get my TS-480hx situated in the cab, not many places to hide stuff in these DCSB tacos.
Neat looking installation Terry.
I can think of two reliable ways to compare antennas.
1. Have your truck out in open country, and place a buddy with a field strength meter a fair distance away, perhaps 1/4 or 1/2 mile. Transmit at low power- just enough to get mid-to 3/4 scale reading on the field strength meter. Swap antennas.
2. Essentially reverse situation, but use a Grid Dip Oscillator or Antenna Bridge to transmit, and use the radio S meter or calibrated ear to compare.
I retired my old noise bridge and purchased a RigExpert AA-30. I decided to wind a couple of shunt coils to help wear the new off of my new tool. For the 20 meter Procomm (shown mounted), it took 4.5 turns to raise the resistive part of the impedance from 32 ohms to 50 ohms, at resonance (zero reactance). For a 40 meter Lakeview Hamstick, it took 6.5 turns (shown laying on the counterpoise) to raise the resistive part of the impedance from 22 ohms to 50 ohms, at resonance. I spread the coils to fine tune the match, as shown.
Last edited by NZ5E; 02-12-2013 at 02:47 PM.
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Nice setup, Terry. Where did you purchase the linear actuator? I'm attempting to fabricate a similar tilting mount on top of a motorhome.
I'm not surprised at the different values for hamsticks. However, if you were using a decent quality screwdriver antenna, you'd discover that just one value will suffice for 80 through 10 meters.
Alan Applegate, KØBG