Weather has been so miserable that I had time to write this long article, summarizing data I had collected during past activation and tests.
I promise it’s not chatting and the info will be interesting, but I warn you: this is long!
If you decide to get on, you better grab some snacks and sit comfortably to read on…
Introduction
While looking for an antenna devoted to reduced space summits, I have been testing a homebrew multiband short vertical antennas, using either a 3 or 4 meter fishpole with a wire.
Focusing in the range 7 to 14 MHz, this short radiator requires certain loading to overcome its electrically shortened length.
Its short length produces a capacitive reactance at the base. The most frequent solution is to use a loading coil right at the feed point, that adds an inductive reactance at the base of the monopole, tuning the system and compensating the reduced length compared to the ideal Quarter wave (¼ Lambda) antenna.
I prepared and tested some prototypes and finally decided to move the coil upwards, towards the center of the antenna as this improves the radiation of the vertical, at the expense of an increased inductance required to put the antenna in resonance again.
An activation in a densely vegetation summit: this loaded 4 m vertical saved my day!
As a complementary improvement, I added a capacity hat on top, that contributes to increasing the current flow in the radiator, and therefore has some positive effects:
- The top hat increases the antenna radiation resistance, significantly more than inductively base loading can do.
- The hat provides a complementary loading, therefore increasing the resulting electrical length of the antenna
- It reduces the required inductance in the loading coil, decreasing a bit some of the associated losses in the coil
- The vertical radiator carries a higher current, as the cap hat works as a storage device (capacitor), in combination with the ground radials. The resulting radiation of the vertical segment is increased due to that greater RF current.
Meanwhile, the wires of the top load, arranged symmetrically, make the currents flow in opposite directions so that they cancel their RF field.
Scheme of adding a cap hat, enhancing the resulting RF current flow:
Using a full length quarter wave is a better performer, but if we plan using a shorter antenna for limited space situations, then, using a cap hat, placing the tuning coil upwards and setting the ground radials as best as you can is a way to get a better signal than taking a mere short whip and use it careless.
I’ve been using cap hats successfully in many summits with good results.
Cap hat, testing several designs
In the past I built different caps. I don’t want a heavy hat, and I therefore used thin brass tubes and also light copper foil.
The general rules for building cap hats are:
- The more surface the higher capacitance obtained.
- You can use as little as two opposed spokes or increase its number. A symmetric & radial design is the common choice although some asymmetry is tolerable.
- You can add an outer peripheral rim, that increases the capacitance a bit.
- If you add more spokes the capacitance increases but there’s no need to put a bunch of them because, at some point, adding more diminishes the increased capacitance per wire, due to the mutual interaction between the near spikes. That’s why a radial structure (easier to build) delivers a similar capacitance than a solid disc.
I decided to run a few measurements to evaluate how the top loading influences on the antenna.
1) Resonant frequency shift test
Is there a large variation in frequency when using a cap hat?
To see the influence of top loading a short vertical antenna I tried a worst-case scenario, that is, a short radiator and reduced ground system.
- Vertical radiator: 1,6 meter wire on a glass fiber pole. This length is similar than short whips.
- Loading coil: not used in any case.
- Ground radials: 2 x 2,5 meter radials lying on ground. That’s a poor ground indeed.
- Top cap hat: I tested the antenna without a cap to have a reference value, and then tried up to 5 different cap hats, from small to big. More details are in the chart below.
With such a short radiator (1,6 m) you could expect antenna resonance is too high, far from useable in HF bands. Let’s see if a cap hat makes things better.
Resonance of 1,6 m vertical without top loading: 34,7 MHz, impedance Z = 38 ohm.
Now let’s add cap hats one by one.
- C Square: made with 5 mm wide copper foil.
- D Decagon: made with 10 mm copper foil
- E Hexagon made with thin Ø2mm brass tubes and a light wire for the outer rim.
As you can see, adding any cap does have a positive influence, shortening the resulting electrical length. Even something as simple as case B (two opposed short spikes), does shift the frequency down.
Better results are achieved as the hat forms a square or rounded surface. See a picture showing the tested hats refs. C, D & E:
Looking at the frequency shift, the E hexagon does the job better than the thick spikes of the D Decagon: that suggests it’s more relevant the area covered (a bit wider for the E hat) than the thickness of the spikes. That’s a good feature as a lighter hat can be made with thin tubes, helping to reduce the side wind loading on the vertical radiator.
The last case, F hat, was done just to get a bigger the top loading, despite from a practical point of view such 2 x 150 cm horizontal wires of my F hat are not a real solution for a mountain top operator. To do this test I extended a rope from two side supports and then hung the 2 x 1,5 m wire stretched; this is something complicated for a mountain top but was nice to test.
This F hat resembles the old T antenna design, frequently used by broadcast MF stations.
See the top parallel wires from the broadcast early AM station WBZ in Springfield MA, 1925 (photo from Wikipedia)
There’s more info on T antennas at the end of this article.
A final note to this set of tests: all the measurements using this 1,5 meter radiator with hats had a good impedance around 50 ohm and very good SWR values, but the resonant frequency without a loading coil were still far from useable for ham bands, except on 28 MHz.
I decided to do some additional testing trying to make this top loaded antenna useable further down in frequency.
2) Testing hats combined with loading coils for 14 MHz: influence in the loading coil.
The hats offer a good contribution to this kind of short antenna, but what could I expect when using a ham frequency of say 14 MHz?
1,6 meters of radiator is very compromised. Is the required loading coil too big? What’s the benefit of using a hat in 20m band?
I ran a second batch of tests to check the variation a hat causes to a base loaded coil. Bear in mind I just tested 14 MHz. If I wanted any lower band the hat contribution could be different, presumably lower…
I just ran two hats for such test, model D-Decagon and F-T Hat (2 x 1,5 meter).
Okay, the good news is you can use this short 1,6 m radiator as an antenna for 14 MHz and the required loading coil inductance is manageable.
Again, using a hat made things better, reducing the required coil roughly by half, moving from the 8uH without a hat to 4,4 uH when using a normal hat.
Oh yes, the T hat made the inductance even smaller, but we agreed it’s not a practical solution.
3) Elevating a short vertical, hat & coil loaded
An additional problem arise with the antenna as seen in the previous test. The impedance for 14 MHz wasn’t that perfect, and you get about Z 53 + j30. That makes SWR around 2:1.
You could still use the antenna, but you have a slight RF reduction due to SWR. Is there any way to revert to a best SWR?
Yes, there’s an easy way to make things better at low cost: raise the bottom of the antenna.
If you’re using such short antenna, you could simply elevate the feed point and the whole antenna 1 meter over ground.
This also makes the ground radials slope a bit then lie on ground influencing the coupling at the base. Even if you’re not using a cap hat the situation will get better with a semi-elevated ground system.
Another test was carried out for this situation, now using 4 x 2,5m ground radials.
It is confirmed that in any case, elevating a short antenna and having semi-elevated ground radials improve the impedance and, therefore, the antenna matching SWR. No evidence of a significant inductance variation was seen in the other hand.
This test help understanding the good results some pedestrian portable operators achieve with their hand-held HF transceivers using a short whip of similar length to my test, which they operate at some height over ground, while trailing a single radial (semi elevated).
4) Is there a way to squeeze more performance? The middle length loaded vertical
Say you want to get some more RF out from your antenna: what else could I do?
Apart from using some more power (rise it from 5 to 10 watts then get +3dB extra …) the obvious choice is to enlarge the vertical radiator.
I wouldn’t use a short 1,6 meter whip in 7 MHz, where I can expect a marginal performance losing many dB compared to a true GP antenna, but, what if I increase the length and use a 3 meter, or better, 4 meter radiator, improving radiation also in other bands?
That’s what I’ve been using in some of my SOTA activation in 2025, and I must say the antenna performed well, running between 7 to 21 MHz. I know its performance is best in the high bands but in 7 MHz I can still grab a decent number of QSO or jump for a quick S2S.
This is not my permanent antenna choice, but it delivers similar results compared to my regular EFHW, which requires much more space to deploy.
What about the required inductance for a 3 meter vertical radiator in 14 MHz? Okay, that’s my final test here (again 4 x 2,5 meter ground radials, but this time the antenna was non elevated over ground):
Tests confirm using the hat there’s a reduction in the center loading coil, so the antenna is slightly loaded thus having better performance than a shorter 1,6 meter radiator.
If I wanted to be perfectionist, I’d elevate the antenna and add another set of radials to get 8 or more on ground. If radials are short, I could also add capacity loading to their ends.
From a practical point of view, this extra effort is something I won’t do as I’d lose the ease of setup.
5) Carrying a hat with no pain: the EA2BD folding hat proposal
I carried my 3m or 4m poles many times in 2025, completed with the coil and the capacity hat. The hat was assembled and didn’t fit inside my rucksack, so I usually hung it outside my pack, like this:
That worked okay but I had to care not to damage the hat when walking in dense vegetation, as branches can grab & pull to the hat, bending the cap spikes or even breaking the outer rim wire. Such issue happened to me a couple times.
What if I prepare a collapsible design reducing its size for transportation purposes? Think & done, this is it, probably the first ever folding ham Cap hat:
How to build it? I took these parts:
- Cardboard to have a solid form
- A decagon shape to create the spikes
- Adhesive copper foil, 10 mm wide (although 5 mm would work the same)
- An expired plastic card and a neodymium magnet
- Thin wire and 3 mm bullet banana
- Wood square dowel, 2 centimeter side
- Scissors & Dremel tool
First, I draw the decagon in the cardboard and removed the inner material with a cutter. Then I added the adhesive copper foil:
I tin solder all corners.
Then added some plastic squares in both sides of the folding axis, drilled a 3 mm hole and used a nylon tie as a hinge.
I trimmed the plastic card a bit, added the neodymium magnet that is in charge of holding the cap hat open. Then soldered two connecting wires to both sides and glued the square wood dowel, after drilling a 5 mm hole to put it on top of the fishpole mast.
This is a top view and see it open, the wire is connected by a 3mm bullet banana.
I protected the cardboard edges with a thin layer of acrylic paint, to withstand wear and a bit of moisture, despite it’s not really watertight.
The easy whip version
In recent times I’ve seen many portable ops using short whips, thanks to the good solar conditions of the current cycle. I’m a bit surprised that none of them seems to be using a cap hat. A cheap and dirty cap hat for whips can be done soldering some short lengths of enameled wire to a crocodile clip: easy to transport and clip on top of a whip!
LY2H, Linas, has a nice video on that, using some solid copper wire.
A look back to the origins: the Marconi era
There are little articles published for cap hats. We can just dig now a bit to get back to what originated its invention.
A look to the archives reveals the capacity hat were originated at the beginning of 20th century, where the broadcast transmitters used very low frequencies.
The T antenna was then born to overcome the difficulty of erecting a true quarter wave vertical for such VLF/LF radio waves. This antenna comprises a vertical radiator and some horizontal long wires at the top, suspended between supporting towers, creating a T-shape.
As we’ve seen, such configuration increases its effective electrical length. Radiation comes from the vertical radiator only, providing a vertical polarized omnidirectional pattern.
The horizontal wires of the top hat don’t provide significant radiation. They extend outwards symmetrically, experiencing symmetrical currents flowing in opposite directions, leading to complete cancellation of their far-field radiation contribution.
The top-load capacitance increases as more wires are added, so several parallel horizontal wires are often used, connected together at the center where the vertical wire attaches.
An optimum efficient top loaded antenna can deliver 2 to 4 times higher power, thus 3 to 6 dB gain. To achieve that level the design has to pay attention to the ground radial design.
T antennas were built early, due to the low frequencies used at that time. Marconi had a T antenna stretched between the masts of his vessel Elettra:
Nowadays, there is still a use of T antennas for broadcast stations.
One example is the DCF77. This is a 50 kW time signal transmitter located in Mainflingen, near Frankfurt, at 77,5 kHz. The antenna top hat is up at 150 meter high.
Some more references on this subject are here:
- The ARRL antenna handbook
- https://en.wikipedia.org/wiki/T-antenna
- https://en.wikipedia.org/wiki/Umbrella_antenna
- https://en.wikipedia.org/wiki/Mast_radiator
- https://grokipedia.com/page/T-antenna
Conclusion
It’s a pity Marconi didn’t have the chance of activating for SOTA.
Let me make a guess: do you think he would have been like this in a mountaintop? That antenna fits quite well with this article…
He would have needed a horse to bring his QRP furniture up the hill !!
73 Ignacio EA2BD
