Topband: 160m T-loaded vertical antenna

[Jo,YC0LOW]

Last weekend, I finished installing the T-loaded vertical antena using the
18m Spiderbeam fiberglass pole. It has 3 wire spokes for top load, each at
15 meter long. Also, 3 elevated radials at 1/4 wavelength long in the height
of 3 meters each. The elevated radials are still bended here and there
according to my limited land-lot area. The SWR 1,6:1 at 1825 kHz. The lowest
1:1 is at 1800 kHz. I think I must pull out radials in straight-and-level
form to obtain the SWR match and resonance (at 1825 kHz) and will see if the
radial wires would need trimming.

I am using coax Andrew 7/8 inch about 21 meters in length as feeder line. My
shack is in the first (not ground) floor. So, both (coax and radials are
elevated). I am using home-made UNUN dual impedance (12.5 & 24 Ohms) a la
W2FMI

This is my first time to build vertical antenna for TX on topband. Would be
very grateful to hear any advises on what to do best to optimise the
antenna. TIA. Also, I'd like to express my gratitude to Bjorn, SM0MDG, for
his suggestions and guidance in making this antenna.

I hope I can continue to present YC-land on topband. 73 de Jo, YC0LOW

[w2pm]

YC0LOW Writes:
-----Original Message-----
From: Jo,YC0LOW <yc0low at gmail.com>


Last weekend, I finished installing the T-loaded vertical antena using
the
18m Spiderbeam fiberglass pole. It has 3 wire spokes for top load, each
at
15 meter long. Also, 3 elevated radials at 1/4 wavelength long in the
height
of 3 meters each. The elevated radials are still bended here and there
according to my limited land-lot area. The SWR 1,6:1 at 1825 kHz. The
lowest
1:1 is at 1800 kHz. I think I must pull out radials in
straight-and-level
form to obtain the SWR match and resonance (at 1825 kHz) and will see
if the
radial wires would need trimming.

I am using coax Andrew 7/8 inch about 21 meters in length as feeder
line. My
shack is in the first (not ground) floor. So, both (coax and radials
are
elevated). I am using home-made UNUN dual impedance (12.5 & 24 Ohms) a
la
W2FMI

This is my first time to build vertical antenna for TX on topband.
Would be
very grateful to hear any advises on what to do best to optimise the
antenna. TIA. Also, I'd like to express my gratitude to Bjorn, SM0MDG,
for
his suggestions and guidance in making this antenna.

I hope I can continue to present YC-land on topband. 73 de Jo, YC0LOW


W2PM Replies:

Jo - how much better a match are you lookiing for with 1.6:1?? ALso,
are you using a current balun choke at the feedpoint at the base? Very
important to avoid losses with the raised radials and coax coupling to
the earth unless they are high, over 15 ft.

73

[Jim Brown]

Post by Jo,YC0LOW
very grateful to hear any advises on what to do best to optimise

Hello Jo,

We are very encouraged that you are working on your station for
topband. When I am on the air around your sunset or my sunrise
(currently 1250Z), I often am able to hear and work stations from
your part of the world.

I have an antenna similar to yours, but the vertical part is
taller. It works quite well. I have four pieces of advice.

First, you need MANY more radials. Radials that are only 3M off
the ground are NOT elevated radials for 160M -- because they are
only a tiny fraction of a wavelength above ground, they act more
like ground-mounted radials. Follow the advice of all antenna
books to add as many as possible, with 60 being a target. Don't
worry about length, don't worry about direction, don't worry about
straight. Just follow the simple rules -- more is better, and
copper close to the antenna is more important than copper distant
from the antenna! And feel free to put them on the ground if that
makes it easier to install them. There is no benefit to raising
them to 3M. (There WOULD be a benefit for a 20M antenna).

Second, do NOT use SWR as an indicator of antenna performance. It
is only an indicator of the impedance match. Your feedline is FAR
too short (only 1/6 wavelength) and far too good to be worried
about that. Any good antenna tuner in the shack will match it to
your transmitter, and many transmitters will be happy with 1.6::1
without a tuner. Rather, I would simply concentrate on reducing
the ground losses (more radials).

Third, consider eliminating the UNUN. Depending on how it is
built, it may be contributing loss. Again, the mismatch to the
line is too small to worry about in such a short transmission
line, provided that you are able to match it to the transmitter
with a tuner or the transmitter can load it. To determine if the
UNUN is lossy, transmit for a long time, then stop transmitting
and go feel the UNUN to see if it is hot. If it is, eliminate it.
If it is not, it is fine to keep it.

Fourth, you should also try to work on some sort of receiving
antenna. You can't work what you can't hear. A Beverage or a loop
can help a lot. There are many QSOs in my log that wouldn't be
there if I didn't have Beverages.

73,

Jim Brown K9YC
Santa Cruz, CA

[k2kw-8]

Hello Jo,

Post by Jo,YC0LOW
Last weekend, I finished installing the T-loaded vertical antena using the
18m Spiderbeam fiberglass pole. It has 3 wire spokes for top load, each at
15 meter long.

Congratulations on your new antenna. N6BT, KE7X and I installed a very similar antenna for the 2007 CQWWCW contest in C6A (C6ARR on 160m). It should be a very good antenna for you.

Our installation was a 55' tall vertical with 4 top-hat loading wires (also 55' long), which came down at rather steep angles due to the limited area we had to work with. We also installed 2 elevated radials, and then laid down about 20 ground screen radials of various lengths. the vertical was mostly 2" thin-wall tubing, tapering to about 1" tubing at the top.

Without any loading coils, it resonated somewhere in the 1.920 MHz range, which is VERY good for such a short antenna. A relatively small amount of base loading was added to bring the antenna to resonance. The base loading coil was made with 1/4" aluminum tubing. We use a small base loading coil on DXpeditions to make tuning in the field a bit easier. It would be better if you can resonate your antenna using the top hat wires only.

Post by Jo,YC0LOW
The SWR 1,6:1 at 1825 kHz. The lowest
1:1 is at 1800 kHz. I think I must pull out radials in straight-and-level
form to obtain the SWR match and resonance (at 1825 kHz) and will see if the
radial wires would need trimming.

For an electrically short antenna like this, this SWR curve seems too wide. I would expect a 2:1 SWR width on the order of +/- 25 kc. Your 50 kc 1.6:1 SWR bandwidth would indicate some losses in your system. I would suspect your radial system (first), or your main vertical radiator width.

To check your radial system, look at the feedpoint impedance: a short vertical like this should be in the 10 ohm range (check without your W2FMI matching transformer). If the impedance is higher, like in the 30-60 ohm range, that is a key indication of too much ground loss.

If so, one way to reduce the ground loss and increase efficiency is to add a bunch of ground radials and do not connect them to the elevated radials or the vertical. I would start with maybe 12 ground radials, and then re-check your SWR curve. If the SWR curve gets narrower AND the feedpoint impedance drops (both of which are signs of a more efficient short vertical), that will again tell you there is too much ground loss with the 3 elevated radials. Hopefully you can leave the ground radials in place, and add more if possible. I guess you could try adding another elevated radial, but I would first suspect your existing radials have too much coupling to ground.

If you are using a wire for the main vertical radiator, that too will reduce efficiency. Adding 2-3 more vertical wires around the sides of the pole and connecting them at the top and bottom of your pole will effectively increase the diameter of the vertical radiator.

Another efficiency point for elevated radials is the width of the wire. I would use at least #12 gauge if possible. Using a small diameter wire will reduce efficiency, and limit how much power you can run. Modeling also suggests that using really large diameter wire (or tubing) for the first 40-50' (if I recall) of each elevated radial will also increase efficiency. This could be a good use for that old coax you have laying around.

For both the main vertical, and the elevated radials, the wider the radiator for the main current part of the antenna, the better the efficiency will be.

Post by Jo,YC0LOW
This is my first time to build vertical antenna for TX on topband.

You are off to a great start! I would first look at the feed point impedance - I think you have too much ground loss.

Best of luck, Kenny K2KW

P.S. the antenna we installed in C6A crushed the QRP world record on 160m in the CQWWCW contest. We did have the advantage of having the antenna on a small bluff about 20m from the ocean with water path to northern JA, USA, EU, and northern AF). But the best QRP QSO C6ARR/N6BT had was over land to VK6HD. This was QRP! Mike sure has great ears.

[Rick Karlquist]

Post by k2kw-8
Our installation was a 55' tall vertical with 4 top-hat loading wires
(also 55' long), which came down at rather steep angles due to the limited

Modelling this configuration shows that beyond a certain length,
making the top hat wires longer is counterproductive. The limit is roughly
where the tips of the wires are at half the height of the vertical.
It may seem good not to need a loading coil, but you are better off
using a loading coil instead of making the top hat come too close
to the ground.

Rick N6RK

[k1um]

I concur adfdfing morde radials. You may also consider a quarter wave shorted stob at the base of t he antenna... (assuming you dont intend on loading your 'T' on any other band

It will do several things - the most important being lowering noise


Ken K1UM

...

[RK]

Greetings Jo,
Congratulations on your new Antenna!

In your post you said your new vertical has SWR was 1:1 at 1800 and 1.6:1 at
1825 kHz. This would indicate that the top-hat is a bit to long. My personal
suggestion would be to tune the antenna so that self resonance occurs at 1900
kHz. At this point it would only require approximately 2uH of inductance to
move the center frequency to about 1830 kHz.

With a proper ground your 1/8 wave electrical height vertical will present a
very good match to 50 Ohm coax through your 1:4 Unun transformer from 1800 to
1870 kHz. Please note that as you improve your ground system you can expect
the SWR to most likely increase as you reduce the ground losses in that
return path. The best way to deal with that is to use the W2FMI 5 ratio or 9
ratio ultimate match Unun if you are concerned about SWR. The good news at
this point if your SWR does change you are reducing those nasty ground losses
which are probably eating up 30-40% of your transmitted power.

If you should choose to use a little base loading to easily move the resonant
frequency lower use a large diameter (4 in) and ratio of 2-3 turns per inch
low loss inductor of copper or silver plated copper wire of at least 10 gauge
or perhaps 1/4 inch copper tubing, as the feed point is also the point of
maximum current. This can easily be done on a piece of Sch 40 PVC pipe and
can also be your base insulator.

So far you have done a great job of minimizing the losses in the radiator. If
you are using the proper mix ferrite core and have would your Unun per
Jerry's directions you will have a very low loss impedance transformation of
less than .1 db. Do Not compromise your antenna performance by using a tuner.
Why throw away at least 10% of your power in external losses not even
associated with the performance of the antenna or it's ground system?

Again good luck and see you on top-band. Short antennas do work very well
indeed by taming losses.

see http://members.cox.net/midnight18/



73 and Best DX