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When it comes to discussion of 630m, the topic of antennas seems to top the list. One of the easiest ways to enjoy what 630m has to offer is to try and utilize a low band antenna that may already be in place. An 'inverted-L' for 80 or 160 can be readily bottom-loaded and with a few radials, can provide a good starting point ... but with a little additional work, its efficiency can be easily improved by expanding the top horizontal (top hat) section.

Jim, W5EST, has posted an interesting description of the pros and cons of the 'top hat' in a recent KB5NJD daily 630m report. Those thinking about getting on the band or those considering ways of improving their present antenna might find the information helpful.

Top Hat Advantages:

Higher EIRP comes from a more nearly uniform current distribution all the way up a TX vertical. https://en.wikipedia.org/wiki/T-antenna . But remember that adding top hat doesn’t help you if your license is subject to a legal limit EIRP that’s reached by your station already.

A vertical without a top hat has no current at its tip, meaning the upper part of a hatless vertical is inefficiently used.  Average RF current for a hatless short vertical is only half what an RF ammeter shows at the antenna base.  Top hat lets a shorter vertical antenna yield same total radiated power TRP by increasing its degree-amperes, as discussed March 31, this blog.

2 amperes of 630m RF base current in a 10° tall hatless short vertical can give 10 degree-amps (2 x ½ x 10°) and yield 15 degree-amps with an ample top hat.  A top hat can increase average RF current by about a quarter to half, which could as much as double the TRP.

Top hat increases antenna system capacitance. You get more flexible QSY by decreasing the system Q.  SWR increases rapidly as your frequency departs from antenna system resonance, see graph Feb. 10, this blog. With lower system Q the SWR doesn’t increase so rapidly. Then you can QSY temporarily a little way without retuning or by just retuning a little in the shack instead of outdoors at the ATU.

Decreased Q somewhat lowers antenna voltage KV from antenna base to top hat. On 630m Q = (2π 475)L/R by definition and Vantenna = 1.4 Q  P / I  <  Vbreakdown.  See Jan. 16, this blog.

Top hat wires can be symmetrically or asymmetrically positioned to give approximately similar capacitance whichever way.   I’ve not modeled the effect of a top hat on the azimuth and elevation antenna patterns of an electrically short vertical.  I don’t think the effect is very significant. But if you know a link or some better information about this, let us know.

If your radials have extended way beyond the extent of a small top hat high above, then providing longer top hat conductors above the radials can more efficiently utilize the radials.   If the radials mostly go in one or two directions, then for highest antenna system capacitance the hat wires should extend in those directions to couple best with the radials.  Your experience may suggest this last is not too important, especially if you have a perimeter conductor and/or several ground rods and your soil has favorable conductance.

Another top hat advantage is that top hat conductors are compatible with structural support and stabilization for the very top of an MF/LF vertical antenna.  You get added degree-amperes–and steadying at the top to boot.

If the top hat slants upward, its system capacitance contribution is somewhat decreased compared to a top hat of same length horizontally, but the vertical slant contributes radiated power. Depending on the arrangement of antenna and trees on some properties, using a shorter vertical with an upwardly slanting asymmetrical top hat may make the antenna system both easier to guy and less obvious to neighbors.

Putting in a top hat or improving a top hat increases the degree-amperes of a short vertical mainly by distributing the same RF amperes more uniformly.  Adding more radials and longer radials decreases the antenna system resistance and increases the degree-amperes of a short vertical mainly by increasing the RF amperes of antenna current itself.

Top Hat Disadvantages:

A top hat obviously requires outdoor work to construct or revise it.  You may be able to simply increase your transmitter power TPO more conveniently than to do the outdoor work.

A top hat needs to extend more or less horizontally from the top of the vertical, although the angle is not too critical within +/-45°.  Distant supports for the top hat at that top level may be unavailable or expensive and inconvenient to provide. If the top hat were attached to the vertical below the top of the vertical, the otherwise radiation-beneficial top segment of the vertical becomes mostly unused.

If the top hat slants quite steeply downward, its effect on system capacitance may be a wash– more capacitance by closer approach to the ground and less capacitance because same length top hat conductor extends less outward over the ground below. That defeats a reason for putting up a top hat in the first place.

Moreover, if the top hat slants steeply downward, then vertically downward RF current in the top hat cancels part of the radiation from the vertical antenna and at least partially defeats the improvement in vertical antenna current uniformity that the top hat is intended to confer.

A long top hat may not fit on the available real estate.  Even if it fits, it may add to visibility as far as difficult neighbors are concerned.

Adding a top hat means you need to retune the ATU after the addition. But so does improving the radials or just about anything else you do.

Top hat conductors add more weight on a vertical than lighter-weight guying does.  The weight of the top hat likely adds to the support demanded of the antenna base.  If you put downward-slanting top hat conductors under tension at their far ends to keep them from drooping in the middle, then a lot of that tension will be imposed on the vertical too.   That can produce a buckling force on the vertical which may call for additional guying halfway up the vertical.

Top hat conductors convey a declining but substantial RF current along their length.  That involves I2R losses in the skin effect resistance of the top hat conductors. However, if your earth resistance is high or your radial/grounding system is not very elaborate, some loss in the top hat probably does not decrease the RF amperes of antenna base current very much at a given TPO compared to the improvement in radiation TRP that the top hat gives you.

If skin effect resistance losses in the top hat are significant compared other losses in the system, reducing top hat losses generally means more conductors or heavier conductors in the top hat.  That translates to more weight for the whole system to support.

A top hat translates KV of antenna top voltage to its ends. If the top hat extends all the way to leaf cover of trees or shrubs, unexpected sparks might jump to them in quiet weather, or in windy weather, or sometime when such trees or shrubs grow nearer to the top hat end(s).

Generally top hat advantages outweigh their disadvantages so long as you plan intelligently. Please tell us your experiences with top hat advantages and disadvantages!”

A G3XDV LF top hat

Jim often adds an interesting op-ed piece to the KB5NJD daily report and sifting back through the past few weeks will provide some great 630m 'food for thought' bed time reading!

Those of you that have been following the recent set of four YouTube videos posted in my recent PAØRDT Miniwhip Shakedown blog, regarding the care and feeding of the PAØRDT active e-probe antenna, may be interested in a fifth video in the series, posted by Mike in the U.K.

PAØRDT's recent modification of the isolating transformer, in the miniwhip coupling unit, provides much better immunity to common mode noise-coupling in the HF bands. Mike shows the installation and operation of the new unit along with some additional grounding out at the antenna.


Those of you that are looking for a possible solution to relatively good reception on LF or on the 630m band, might find the answer with a carefully thought out installation such as the one shown in Mike's video series.

Although living in the suburbs often means dealing with a lot of noise, particularly on LF / MF, Mike's system shows that excellent low noise reception is indeed possible. It is important to 'probe' your backyard with the antenna itself in order to find the quietest location for installation.

More information on the PAØRDT miniwhip and other low-noise LF / MF antennas may be found in previous blogs here.

Even though the long range forecast for Saturday, March 19th is calling for rain and drizzle, I am contemplating doing another NPOTA activation.  The weather this time of year is always changing, so I am hoping that next Saturday will actually turn out nice.

This time I have a hankering to go on up to HP28, Morristown National Historical Park, which actually consists of four different areas:

1) The Ford Mansion
2) Washington's Headquarters Museum
3) Jockey Hollow
4) Fort Nonsense

The best area for operating without causing too much undo commotion to anyone else, is from the parking lot at Jockey Hollow.  Jockey Hollow is where the Continental Army wintered in 1779-1780 - a winter which turned out to be way more severe than the previous winter, which has become better known as "The Winter at Valley Forge".

That's where Dave KD2FSI activated HP28 back in January (and where I logged for him as he handled his really first big HF pileup).  I noticed some pretty tall trees right next to the parking spots (the parking lot is big), so maybe I can quietly throw my PAR END-FEDZ 40/20/10 into one of them.  Of course, the main bands for operating will be 40 and 20 Meters.  As is always the case for W2LJ, operations will be primarily CW - but I am thinking of perhaps even doing a little QRP SSB if activity on the CW bands gets lax.  I'm not sure if HP28 has been activated via CW. Maybe I can be the first.

This weekend, if I get the time, I would like to finally experiment with building a magnetic loop antenna for 40 - 10 Meters.  I have some coax in the basement that's too short for anything else and would serve well, I think.  I have a 365pF variable cap that I got from eBay.  I have been reading some articles lately and it doesn't look like building a rudimentary loop would be all that difficult.  If I have success with it, maybe I can eventually come up with a homebrewed version of the AlexLoop WalkHam model that I can use for portable ops. It might prove to be a viable option when I want to be away from the Jeep, and yet I can't hang an EFHW, or doublet.

72 de Larry W2LJ
QRP - When you care to send the very least!

Even though the long range forecast for Saturday, March 19th is calling for rain and drizzle, I am contemplating doing another NPOTA activation.  The weather this time of year is always changing, so I am hoping that next Saturday will actually turn out nice.

This time I have a hankering to go on up to HP28, Morristown National Historical Park, which actually consists of four different areas:

1) The Ford Mansion
2) Washington's Headquarters Museum
3) Jockey Hollow
4) Fort Nonsense

The best area for operating without causing too much undo commotion to anyone else, is from the parking lot at Jockey Hollow.  Jockey Hollow is where the Continental Army wintered in 1779-1780 - a winter which turned out to be way more severe than the previous winter, which has become better known as "The Winter at Valley Forge".

That's where Dave KD2FSI activated HP28 back in January (and where I logged for him as he handled his really first big HF pileup).  I noticed some pretty tall trees right next to the parking spots (the parking lot is big), so maybe I can quietly throw my PAR END-FEDZ 40/20/10 into one of them.  Of course, the main bands for operating will be 40 and 20 Meters.  As is always the case for W2LJ, operations will be primarily CW - but I am thinking of perhaps even doing a little QRP SSB if activity on the CW bands gets lax.  I'm not sure if HP28 has been activated via CW. Maybe I can be the first.

This weekend, if I get the time, I would like to finally experiment with building a magnetic loop antenna for 40 - 10 Meters.  I have some coax in the basement that's too short for anything else and would serve well, I think.  I have a 365pF variable cap that I got from eBay.  I have been reading some articles lately and it doesn't look like building a rudimentary loop would be all that difficult.  If I have success with it, maybe I can eventually come up with a homebrewed version of the AlexLoop WalkHam model that I can use for portable ops. It might prove to be a viable option when I want to be away from the Jeep, and yet I can't hang an EFHW, or doublet.

72 de Larry W2LJ
QRP - When you care to send the very least!

A little less than a year ago I summited a mountain, known in SOTA terms as 9431 (it's elevation ASL) with a designator of W5N/PW-019, which put me over 1,000 Activator points and thus qualified me for the Mountain Goat award. This award is one of the most satisfying awards I've achieved in ham radio, including #1 Honor Roll. I did the summit that day with a couple of friends of mine, Fred KT5X (aka WS0TA) and John, K1JD. Both are also mentors to me as I learned the SOTA trade so to speak.

A little less than a year ago I summited a mountain, known in SOTA terms as 9431 (it's elevation ASL) with a designator of W5N/PW-019, which put me over 1,000 Activator points and thus qualified me for the Mountain Goat award. This award is one of the most satisfying awards I've achieved in ham radio, including #1 Honor Roll. I did the summit that day with a couple of friends of mine, Fred KT5X (aka WS0TA) and John, K1JD. Both are also mentors to me as I learned the SOTA trade so to speak.

courtesy: http://www.leeszuba.com/projects/

A recent blog posting of four videos showing the installation of a new (Roelof-made) PAØRDT active E-probe antenna revealed that there appeared to be a difference between his earlier model and his latest. It was noticed that the noise ingression levels were different between them, with one showing less noise on LF than on HF, while the other one behaved just the opposite. The newer PFU has the antenna ground isolated from the receiver's ground while the older one does not. Noise coupling, with the new one was higher on HF and lower on LF, which was just the opposite of what was noted with the older unit with its un-isolated ground ... overall LF performance over the newer system was deemed to be better as seen in the final video here.

The only difference seemed to be in the coupling isolation transformer, used in the power feed unit (PFU), likely similar to the one shown at the bottom of this page.

Roleof's transformers are wound on a Magnetics 0W40705TC toroid, whose high mu ferrite has an Al value of ~8350. I mentioned to Roelof that is seemed unusual that the noise levels would be greater on HF than on LF, with the new transformer, as usually it's the other way around when it comes to noise. The changes noted in the video lead to further transformer testing by Roelof, who indicated :

The isolating transformer consists of two bifilar windings on a high mu core. My guess is that the interwinding capacitance spoils the isolation at HF. I have just tested an isolating transformer with two separate windings opposite each other on the tiny core. Sure enough, this solves the feedthrough at HF and VLF performance is not impaired at all.
I have never given this a second thought as I believed it was specific for my location. Never too old to learn!

I have evaluated both transformer types on my spectrum analyser and
found the following.

The current transformer with a twisted bifilar winding is a
transmission line transformer. In a 50 ohm system, it covers 4 kHz (!) to 200 MHz at -3dB. Which is very good.

The new one, using the same core and separate windings is an other
story. It covers just 4 kHz to 8 MHz at -3dB. At 30 MHz the loss is
13 dB. So, excellent for VLF / LF/ MF, it won't do for upper HF.

I will try other core material and see if a feasible compromise is
possible.

To be continued.

further...

I first tried a FT-37-43 toroid with 10 turns for each winding.
This core is suitable for higher frequencies and I expected it to
work better than my high mu (8300) cores.
Nope, it was far worse and at 30 MHz the loss was already > 20 dB.

I reverted back to my magical cores and reduced the number of turns
to 3. This yielded a nice bandwidth (-3dB)from 50 kHz to 50 MHz.

Used in the mini-whip interface, there is no more a difference
between shared grounds and isolated grounds on either VLF / LF or
HF.

Though the lower -3dB point is at 50 kHz, I still have excellent
reception of the Russian Alpha stations between 10 and 15 kHz.
It looks like this transformer is an excellent compromise for this
purpose.

I have often found that in practice, high mu cores are excellent for
wideband transmission line transformers. They are good for
traditional rf transformers as well!

and:

I just have tested a binocular core, the BN-73-202, available from
W8DIZ.

With two windings of two turns each of insulated hook up wire (to
keep interwinding capacitance low), gives a transformer from 80 kHz
to over 100 MHz. In practice reception down to 10 kHz is still
excellent.

There is no difference between isolated or common grounds either.
The coupling with this binocular core is a little better than with a
toroid. At LF / MW, I don't find any loss at all.

The FT-50-75 will do fine with two windings of 4 turns each.
I have some FT-50-77 cores at hand and will see how they work out.

I am surprised at the wide bandwidth that can be achieved.

and:

I have been testing a FT-50-77 ferrite toroid for use as wideband
isolation transformer.

Three turns on the Magnetics (aka magical) cores, give an inductance
of 60 uH. Used as a wideband Isolation transformer, the -3dB
bandwidth ranges from 80 kHz to 50 MHz.

For about the same inductance, the FT-50-77 core needs 6 windings.
Used as wideband isolation transformer, the lower - 3dB point is 100
kHz, the upper is 12 MHz. At 30 MHz the loss is already 8.5 dB.
Though not suitable as wideband isolating transformer, it still can
be used for the range 100 kHz - 3 MHz.

It looks that for good wideband performance the Al value must be >
5000 and the number of turns should be =< 3.

Interesting stuff and the best thing is that it has really lowered
my noise floor!

A summary posting to the RSGB's LF reflector indicates that the new transformer scheme is providing quieter VLF reception and good performance up to 200MHz

For years, I have been using a rf - isolating transformer to
separate antenna and receiver ground.

This is a home made transmission line transformer, consisting of 11
bifilar turns on a small high mu toroid (AL=8300). The inductance is
1.2 mH. The measured -3dB bandwidth in a 50 ohm system covers 4 kHz
- 200 MHz.

It appeared that the isolation was not perfect, due to the
inter-winding capacitance. I have made a new transformer on the same
core with two windings of each tree turns. The windings are opposite
each other on the core. The bandwidth is now 45 kHz - 50 MHz. The
loss is 1.3 dB.

Despite the raise in the lower -3dB point, reception at VLF is much
improved due to lower noise ingress. See attachment.
The screen runs from 1 kHz - 13 kHz. At the right hand the dashes
from the Russian Alpha system can be seen. The white band is with
common ground. Harmonics of the 50 Hz mains can be easily spotted.
In reality the picture will be better,as the sensitivity of the
PERSEUS drops considerable below 10 kHz.

The level of interference on 380 kHz caused by a plasma TV in the
neighbourhood is also much reduced.


An excellent video demonstrating the dramatic effects of isolating the ground in the e-probe antenna system on LF was posted today by PY3CRX. Marcus used the Magnetics hi-mu core with 5 + 5 turns (~ 390 + 390 uh).


For those not having access to the Magnetics material, it looks like the BN-73-202 binocular core is a good performer, from 10kHz - 100MHz. The FT-75 and 77 material also fair well but with differing bandwidths. Cores are available from Amidon as well as from W8DIZ's Kits & Parts.