Size of capacitance top hat

Do you mean 10pF? 

Anyways, a couple things to note: 1) It would be very easy for the FCC to consider a top hat to be an extension of antenna length. 2) IMO, more important than anything else with a Part 15 antenna is peak resonance, so if you add a top hat, you will have to re-tune the antenna. Likewise, if you change the length of the top hat elements, you will also have to re-tune.

For me, keeping within limits, tuning my bottom loaded copper pipe antenna by coil turns and finishing by length adjustment, resonance occurs at very small tolerance...1 width of a Sharpie pen marker. Any more up or down and it drops off sharply.

There are several posts elsewhere on the internet that suggest that small top hats have a negligable effect on performance.

Sorry, yes i meant pf.
I have constructed the antenna with 2" diameter alloy flagpole sections.
The antenna consists of 2 sections that are pop riveted together, and with the finial+bolt on top makes it approx 102".
This antenna then fits over a thick fibreglass pole( which acts as an internal sleeve), that acts as the insulator, then a further 6' of tube beneath this(with approx 2" gap), which has 40"+ in a ground sleeve, set in concrete.
As the whole setup is fairly rigid, i can fix a reasonable sized capacitance top hat to it.

I am trying to broaden the bandwidth best possible, so i wondered what a 4' diameter top hat would offer over a 2' one ?

Also i will be using 16x 3 metre ground radials.

PS, I live in the UK.

Paul.

Let is know before and after if it is possible and does not create too much extra work. 🙂

Looks like i have hit a big problem.
To test my short flag pole antenna, i laid down 8x 3 metre ground radials on the grass, and connected them to the AMT5000, tuned the output matching for max current, no problem, but the range is very poor.

Approx 14 metres away from the setup i have a steel lattice mast that is approx 25 metres tall with a large vhf beam antenna on top.

I expected this to alter the radiation pattern of the part 15 setup, but not fatally in all directions !

Using a Realistic dx440 portable receiver, range is approx 70 metres in any direction, and in the car it is no better.

Has anyone had experience with this situation ?

It stands out to me that you have so confidentally concluded that the tall mast 14 metres from your low power antenna is the cause of the poor range. It may possibly be the cause of the problem, but I wouldn't suspect it because I think the two structures are far enough away from each other to operate indepentantly.

In my case I connected an indoor AMT5000 to a metal window frame (calling it a Wintenna) and on the outside connected a vertical wire at the top of the window to accomplish the remainder of the 3-metre height, with two ground radials coming from underneath the room out into the yard on one side and into the basement on the other. This gives me a range of about 500-feet...

The wall around the Wintenna contains metal latice for the stucco finish, and increases the capacitance of the AMT5000 output to the extent that I needed to add a capacitor on the antenna line (I think it was 22 pF) so I could tune to resonance.

Maybe you could try adding capacitors by experiment (in-line with the RF output) to see if you can get a better match.

Hi Carl,

Do you mean adding capacitance in series with the output to the antenna, or in parallel ?

Paul.

In series.

Mr. BOARDMAKER I checked the cap in series with my AMT5000 RF Out and it is 20 pF.

Of course other situations very likely require other values.

If this solution works for you, you may do what I did by trying different values until you find that one that puts the tuning range of the AMT5000 into a workable range.

Thinking back, I began by taking a capacitance reading of the metal window (antenna) using a meter, and the result was an astounding 4,000 uF.

The AMT5000, as the Manual explains, is designed to match a 3-metre vertical antenna in an open environment and that's NOT what I have, but the capacitor got me going.

I get the sense from your description that you are closer to having an open environment, so I will guess you don't need anywhere near the value cap that I used (if that's the solution.)

I don't know - five years ago -

I had a tophat on my "about 9 foot

Part 15 AM stick."

It was big.  4 spokes.  Each 2 1/2 feet long.

Mechanically it was a nightmare.  But it

seemed to do quite a bit of good in the range

department.  I made the decision (just for me

and nobody else) that the tophat was a capacitor -

and was not part of the antenna length.

It was a long time ago.  If you disagree - that's

OK with me.  I don't even know if I was right.

A lot has happened in the last 5 years.  I don't

completely remember how I kept the whole thing

up in the air - but it was a lot of fun - that's for sure!

Brooce, WLP

With my current setup, at 1512khz, the am5000 output matching peaks with the inductance at minimum value using s11 jumper, and c1 capacitor at approx 2 turns in.
I assume a series capacitor would be used if was unable to peak the matching ?

If i were to fit a capacitance top hat, i would need to take some turns off the inductor, which would be no problem.

Comparing the amt5000 chart, this antenna is certainly has more capacitance than a straight whip antenna.

I will make the top hat and reduce the turns on the toroid and post the results.

Paul.

Less inductance is a good thing.

I have now fitted a capacitance top hat to the short flagpole antenna, this consists of 8x 2' stainless rod.
Antenna now resonates up 153khz lower in frequency, so before it was 1512khz, now it is 1359khz, with just a couple more turns of the fine tune capacitor in the amt5000.

As i was previously using s11 jumper(min inductance tap), i have now removed 15 turns of the amt5000 inductance, and now with s11 still used, it peaks with approx 5 turns in of the fine tune cap.

Signal wise, it gets out better, with more uniform coverage, a modest improvement, but still far short of what it should be doing.

What is very interesting though is that 190 metres away up the road, the signal peaks very strong at a power supply pole where a mains cable exits the ground up the pole, and flies overhead to a house !

My theory here is that 2 possibilities exist to cause this,

My steel tower 14 metres away is absorbing the transmission and radiating the signal in to the mains wiring through the rotator wiring or vhf coax cable, into the house wiring.
Or the other possibility is the cat 5 supply and audio back to the house from the amt5000 outdoors.
I will disconnect the coax and rotator wiring from the steel tower first, and test.

I have a feeling that the steel tower is the culprit here, and is absorbing the rf ?

Btw i have 2 amt5000's, and a denon amax tuner, the flagpole improves the audio bandwidth, over the wire antenna.
The flagpole with capacitance hat improves it yet again, as well as a wider peak on the amt5000 tuning cap.

Paul.

Your improved result is a message to me, also owner of AMT5000, now it's time to re-think tophats.

I might attach a CD (compact disc) to the top of an antenna as experiment # 1.