CAPACITY HATS FOR HF MOBILE ANTENNAS - testing the DX Engineering "Hot rodz"

What is a capacity hat?

Rather than explain it here, you can find a nice section dedicated to Capacity hats at K0BGs website


The goal of todays experiment: see how much a Cap Hat lowers frequency

(As usual, you can click on any of the photos to enlarge them)

I started with a quarter wave on 10m made up of two screw together sections plus the base of the hotrodz sytem :

 

As  you can see below, it was a bit long and resonated aroud 27 MHz, but since this is just for comparison, it doesnt really matter. 

I then added two 24" sections at the top to make a "T" shaped top loaded antenna.

 

As you can see, it brought the resonance down to 20.4 MHz, a 6.5 MHz drop in frequency.



I then took the bottom 4' section off, leaving the antenna at around 5' long

 

 

I ws unable to measure the resonant point since my analyser only goes up to 30 MHz but we can asume maybe 31 or 32 MHz.

The next step was to increase the capacity by installing another 4 24" rods

 

This brought the frequency down to around 25.6 MHz.

 

 

To see if I could raise the frequency I tried "shortening" the tips. After a few attempts using trial and error, I shortened each tip by approx 6 " (sorry, I didn't measure them) 

 

 And managed to get this final result



 

 



 

 

 

So, am I going to start using cap hats from now on to reduce the height of my antennas without losing much efficiency?

 

The answer in my case is no, and the reason is because of the weight/stability of the antenna system. The mast and hub is heavy duty and makes a pretty top heavy system for using with my puny ball mount on the roof. Not to mention that in my case where the antenna is always installed and taken off every time I park up, this is too long and unwieldy.a process.

 

Where I might use this information is if one of these days in the future I might decide to make a shortened 80m vertical on a spiderpole, for a contest or something. Or, to make vertical dipoles like the old Force 12 Sigma or XR series antennas for using in phased arrays or 4 squares on the dock, where I'd like to keep wire on the ground to a mínimum.

 

 

 

 

 

 

 

 

 

 

 

 

 



 

GROUND TUNING UNITS (GTU) IN A MOBILE

To GTU or not to GTU, that is the question.....

Something amiss in the new car?

Over the last couple of years, there have been a number of expeditions active which I needed for a"new entity", but was unable to work. In spite of doing my best to come on at the right time of day, and in spite of trying all the DXing tricks of the trade, I just wasn't making it through the pileup. Many of my local friends were making the contacts, so, I started thinking that maybe something was up with my station. I started suspecting that when I changed cars, something went amiss with the new antenna installation, or that there was maybe a problem with the antenna itself.

Parallel to this, I was reading a lot on the Real HF mobile yahoo group about how people were using GTUs (ground tuning units) to enhance their pedestrian mobile signals. Many of them were reporting good success, and I know that at least one station used a GTU on his car as well.

GTU theory
So, what is a GTU? Well, basically all it is is a device, similar to an Antenna Tuning Unit, which uses an inductor and a capacitor in series to maximise current flow in the radial system and therefore make the ground system less lossy and hence the whole antenna system (radiator+ground/radial) more efficient.

Attempt nº1
My first attempt at trying a GTU system was last year. I purchased an MFJ 931 artificial earth, which in theory does the job just fine. In order to isolate the antenna from the car body, I used a magmount on top of a plastic book binder. I connected the base of the magmount to one end of the the MFJ, and then the other end to one of the bolts on the antenna mount which is grounded to the car body. Result? Nowt. Zilch Nada. Couldn't get a current reading at all, anywhere, anyhow. Thought the MFJ was dud and even though I opened it up and couldn't find anything wrong with it.

Back to the drawing board

More DXpeditions came along and whilst some I was able to work, others continued to elude me. So, back to the drawing board it was.

I contacted GTU guru Dave G4AKC, who is a very well know signal on the bands, either pedestrian mobile or bicyle mobile. Dave wrote back to me to say:

Quote:

The first thing is to ensure the antenna is resonant against a resonant radial(s), this is best done using an antenna analyser and of course with the antenna off the car and the antenna length set to provide 50 ohms at resonance as you would expect. I usually resonate my vertical antennas at home on the top of my flat garage roof, so the radials are elevated and are located about a quarter mile from the sea, however I don't think it will make a lot of difference unless you are in the fresnel range of the sea where we get the enhancement to our signals.

 

The antenna is then refitted to the car and the antenna length is not changed, I know you are using the MFJ-1979 so I guess a mark to identify the length at resonance.

Its essential that the radio is isolated from the car body with regards to an RF path and so the radio needs to be physically kept off the car body and the DC negative line is taken via a simple toroid with several turns of this DC negative line wrapped around it and this is sufficient to prevent a direct RF path back to the chassis. As long as the toroid includes the negative return feed it shouldn't make any difference if you toroid both the + and the -ve lines, however you will need to get at least a couple of turns around the toroid for it to work effectively, so a large toroid will be required! You will have to toroid the amplifier negative line as well, but the clip on large toroids are fine as long as you can get a few turns around them to produce a high impedance path to the car body.

 

The braid of the coax feedline at the antenna end is routed via the GTU, if you are using a mag mount its easy to run a wire from the frame of the mag mount back to the GTU and then returned to the car body.

If you have a bonded mount then of course this needs to be insulated from the chassis of the car.

I usually tune with a low power carrier so as the ground current is at maximum, this current and tuning point alters depending upon location due to different ground conductance as you would expect and of course different settings for different bands, however generally once you have set the GTU for a particular static mobile location the setting will remain pretty much the same every time unless the location is very tidal.

 

You should be able to produce a considerable amount of ground current when near to the sea, I  can get as much as a couple of Amps of RF ground current at high tide with the bike running high power, so you will definitely know when its working properly, the key is to RF block any direct path to chassis other than the GTU path.

 

Unquote

 

Shopping!

 

I wanted to use the hustler mount on the car, so while checking out stuff on the web I came across these wonderful 45KW insulators by Breedlove antenna mounts. By the way these folks do an excellent line in all things antenna mounting.

 

I also thought it was high time to invest in a field strength meter so bought myself a cheap and cheerful MFJ 802 meter . I've read now in so many places that the best way to tell if antenna A or antenna B work better is simply by seeing if the near field is stronger or weaker. There is a great article by G3TSO where you can read all about this and other questions of resonating HF mobile antennas.

 

Attempt nº 2

 

So, once I got all the bits and pieces together, I went up to my house at La Eliana (where I can get peace and quiet, plus a soldering iron near the car if needed).

Before starting, and although most of the tests weren't using the car body, I followed Dave's suggestion about the toroids and snapped on a few to the rig's negative lead.

 

The first thing I did was try the MFJ 1979 in its usual position, using the untuned car body for earth. Here's how it looked on the analyser for SWR and resonance (reactance X = 0):

(remember you can click any of the images for larger size)

 

Ok so this looks pretty textbook and to be honest I don't remember that it looked like that last time I put the analyser on it.

The next step was to insulate the antenna from the car, and use a quarter wave radial cut to the exact same size as the radiator

This is what it looked like on the analyser:

OK, so it shifted down a bit in resonant frequency, and so obviously at the higher end we now see the L/C factor creeping into the total R. But still, not bad, and on the trusty FS meter (on a stepladder about 2m from the antenna), no noticeable change at all.

Then, the next logical step was to try tuning the radial with the GTU, so thats what I did:

And hey! The GTU Works! We got CURRENT FLOW!! :)

But you know what? Lower FS on the FS meter. Hmm..... What on earth (pardon the pun).

Here's how it looked on the analyser:

 

Again, an even lower resonant frequency and obvious departure from resonance (X = 0) the further away we get from the resonant frequeny.

Finally, I decided to tune up the coax that leads up the ball mount. This is what I was ulltimately aiming for, being able to tune either the cable or the body of the car for improved performance over the untuned car body. Well,. I was able to get decent amount of current to flow through the cable, but at the máximum current, the SWR was sky high on the radio's SWR meter. I didn't take any readings on the analyser for this. And also, the FS meter reading was a fair amount lower than for the previous configurations.

CONCLUSION

I am going to retest the "usual position" next to the water to see if it replicates the above readings.

If it does, the conclusión is that whilst GTUs are obviously good for improving performance of small counterpoise trolleys, bicycles, etc., for a larger metal object like a car, there is no advantage to be had. Unless of course Dave G4AKC can convince me otherwise!