With regard to the version you made - did you account for the loading
effect of the PVC when you measured and trimmed the length of your
radiator?


Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you stated,
the PVC will change the input Z of the radiator. Just about anything
that is too close to the radiator will change things.
According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above ground
is 80-j300. As such, with a single series inductor matching section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.
There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending on
the application, is NOT the best performing radiator.
Gary N4AST

In article .com,
wrote:

Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you stated,
the PVC will change the input Z of the radiator. Just about anything
that is too close to the radiator will change things.

On second reading of that QST article, I admit to being a bit curious.
The K4LPQ version (with a shorted inductive stub inside the radiator,
soldered to it) is clear enough. However, the W9WQ version using a
longer, open-circuited inductive stub wire isn't a straightforward
translation of this, because there's no soldered (or other DC)
connection between the radiator and anything else!

I infer that in the W9WQ version, the stub wire is performing two
functions at once - it's adding a series inductance, and it's also
coupling the RF out onto the radiator in a capacitive fashion. This
would imply that the stub needs to provide a bit more inductive
reactance than in the K4LPQ shorted-coax version, with some of this
reactance cancelling out the radiator's -j300 and the rest cancelling
out whatever amount of capacitive reactance exists between the stub
wire and the radiator.

Am I reading this right, or am I missing something?

[Regretfully it seems likely that both W9WQ and K4LPQ are now silent
keys, so I can't ask for advice from the horses' mouths.]

According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above ground
is 80-j300. As such, with a single series inductor matching section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.

I've seen a number of 5/8-wave antenna designs which deal with this
issue by using something other than a simple series coil. The
commonest approach seems to be to use a coil which is connected in
shunt between the radiator and the ground plane, with the "hot" side
of the coax being fed to a point tapped partway up on the coil. This
approach transforms the radiator impedance down to the 50 ohms needed
to match the coax, and also provides the series inductance needed to
cancel out the reactance. It also provides DC grounding for the
radiator.

http://www.fluxfm.nl/schema/5-8%20go...20radialen.PDF is one
such design. It requires some amount of tooling (e.g. to lathe down
the plastic parts to the specified configuration) but I suspect that a
version could be homebrewed up using simpler materials and methods.
The photos show the way to build the tapped matching coil assembly,
and could probably be adapted to other coil construction methods. I
believe the ARRL Handbook article which adapts a Radio Shack CB mobile
antenna uses a similar tapped coil.

Another approach might be to change the length of the radiator a bit,
to change the resistive part of the feedpoint impedance from 80 ohms
down closer to 50 ohms, and modify the coil to suit. I haven't run
any simulations to see how much change in the radiator length would be
required, and what this change would do to the antenna's gain pattern.

There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending on
the application, is NOT the best performing radiator.

I'll have to look it up if I can find a copy of that edition to see
what they have to say. I agree, in some cases the horizon-directed
gain of a 5/8 isn't what you want. For in-city and in-the-hills
mobile use, a 1/4-wave might give more reliable performance, precisely
because its RF energy is more broadly directed.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
In article .com,
wrote:

Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you
stated,
the PVC will change the input Z of the radiator. Just about
anything
that is too close to the radiator will change things.

On second reading of that QST article, I admit to being a bit
curious.
The K4LPQ version (with a shorted inductive stub inside the radiator,
soldered to it) is clear enough. However, the W9WQ version using a
longer, open-circuited inductive stub wire isn't a straightforward
translation of this, because there's no soldered (or other DC)
connection between the radiator and anything else!

I infer that in the W9WQ version, the stub wire is performing two
functions at once - it's adding a series inductance, and it's also
coupling the RF out onto the radiator in a capacitive fashion. This
would imply that the stub needs to provide a bit more inductive
reactance than in the K4LPQ shorted-coax version, with some of this
reactance cancelling out the radiator's -j300 and the rest cancelling
out whatever amount of capacitive reactance exists between the stub
wire and the radiator.

Am I reading this right, or am I missing something?

[Regretfully it seems likely that both W9WQ and K4LPQ are now silent
keys, so I can't ask for advice from the horses' mouths.]

According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above
ground
is 80-j300. As such, with a single series inductor matching
section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.

I've seen a number of 5/8-wave antenna designs which deal with this
issue by using something other than a simple series coil. The
commonest approach seems to be to use a coil which is connected in
shunt between the radiator and the ground plane, with the "hot" side
of the coax being fed to a point tapped partway up on the coil. This
approach transforms the radiator impedance down to the 50 ohms needed
to match the coax, and also provides the series inductance needed to
cancel out the reactance. It also provides DC grounding for the
radiator.

http://www.fluxfm.nl/schema/5-8%20go...20radialen.PDF is one
such design. It requires some amount of tooling (e.g. to lathe down
the plastic parts to the specified configuration) but I suspect that
a
version could be homebrewed up using simpler materials and methods.
The photos show the way to build the tapped matching coil assembly,
and could probably be adapted to other coil construction methods. I
believe the ARRL Handbook article which adapts a Radio Shack CB
mobile
antenna uses a similar tapped coil.

Another approach might be to change the length of the radiator a bit,
to change the resistive part of the feedpoint impedance from 80 ohms
down closer to 50 ohms, and modify the coil to suit. I haven't run
any simulations to see how much change in the radiator length would
be
required, and what this change would do to the antenna's gain
pattern.

There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending
on
the application, is NOT the best performing radiator.

I'll have to look it up if I can find a copy of that edition to see
what they have to say. I agree, in some cases the horizon-directed
gain of a 5/8 isn't what you want. For in-city and in-the-hills
mobile use, a 1/4-wave might give more reliable performance,
precisely
because its RF energy is more broadly directed.

--
Dave Platt
AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Hi Dave, I'm sure you are reading it right, will look and see if I can
add anything later.
Gary N4AST

Dave Platt wrote:
In article .com,
wrote:


On second reading of that QST article, I admit to being a bit
curious.

I infer that in the W9WQ version, the stub wire is performing two
functions at once - it's adding a series inductance, and it's also
coupling the RF out onto the radiator in a capacitive fashion. This
would imply that the stub needs to provide a bit more inductive
reactance than in the K4LPQ shorted-coax version, with some of this
reactance cancelling out the radiator's -j300 and the rest cancelling
out whatever amount of capacitive reactance exists between the stub
wire and the radiator.

Am I reading this right, or am I missing something?

I took a look at the Smith Chart, and for impedances in this region
of the chart, series L and parallel C is not the way to get a match. In
my version, 80-j300, you need 4.7pf shunt C and .15uH series L. I have
no idea how Fig. 1C in the article managed to get a good match with the
single insulated wire up the middle of the radiator.
I seem to recall a tri-band beam (TA-33 jr.?) that used this type of
matching. Must work, so I guess I am missing something.
Gary N4AST

Am I reading this right, or am I missing something?

I took a look at the Smith Chart, and for impedances in this region
of the chart, series L and parallel C is not the way to get a match. In
my version, 80-j300, you need 4.7pf shunt C and .15uH series L. I have
no idea how Fig. 1C in the article managed to get a good match with the
single insulated wire up the middle of the radiator.

It's possible he isn't creating a full match (with an L network) in
this case. He might just be cancelling out the negative reactance,
using a combination of (series L from the stub, and a bit of series C
from the capacitive coupling between stub-feed and radiator), and not
bothering with a shunt component at all. This would, perhaps, result
in an 80+0j feedpoint impedance and about a 1.6 SWR at the feedpoint,
which would probably end up significantly lower at the other end of
the feedline due to feedline losses.

Or, there might be something stranger going on, with the stub giving a
bit of shunt C to ground (in the PL-259), some parallel L/C inside the
radiator, and six other bits of odd voodoo.

The author says that it ought to be possible to get down to below
1.5:1 on the repeater portion of the band... this suggests that the
design isn't one which "tries" to achieve a true 1:1 match. The WA-2
and similar 5/8-wave antennas using a tapped coil seem to be able to
get down arbitrarily close to 1:1 at their best.

Beats me. Almost makes me want to try building one just to measure it
out and see how well it can work. On the other hand, given the
comments by Cebik and others about the somewhat illusory nature of the
gain advantage of a 5/8-wave, I may just stick with J-poles and
quarterwave ground planes.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Dave Platt"
... On the other hand, given the comments by Cebik and others
about the somewhat illusory nature of the gain advantage
of a 5/8-wave, I may just stick with J-poles and quarterwave
ground planes.
_______________

Just to note that for AM broadcast verticals, the FCC requires a certain
antenna "efficiency" for various classes of stations, in terms of the
minimum ground wave field strength produced per kilowatt of input power to
the radiator.

The FCC field strength minimum cannot be met by "Class A" stations
(basically the 50kW-ers) using a 1/4-wave vertical radiator. At least a
1/2-wave radiator is needed in most cases.

The most common radiator height for Class A non-directional AM broadcast
stations operating at 50kW day and night is 195°. A radiator height of 225°
(5/8 wave) maximizes ground wave field strength at a given power, but also
produces a high-angle lobe that can interfere with the ground wave during
night-time operation -- so rarely is used by AM broadcast stations. The
ground wave field strength difference between 195° and 225° radiators is
fairly small.

RF
Former staff engineer, WJR Detroit -- (Class A, 760kHz)

Richard Fry wrote:
"The most common radiator height for Class A non-directional AM
broadcast stations operating at 50kW day and night is 195-degrees."

I won`t challenge that as I have conducted no survey. WJR Detroit is
shown on the broadcast allocations map book as an unlimited (day and
night) 50 kilowatt Class 1 station.

Class 1 stations operate in a clear channel (this does not mean alone on
the channel) with an assigned power between 10kW and 50kW

A Class 2 station operates in a clear channel with an assigned power
between 250W and 50kW. They must operate so as to not cause interference
to the Class 1 stations. There are 29 clear channels which permit class
2 station operation.

Class 3 stations share regional channels and operate with assigned
powers between 500W and 5kW. Thyere are 41 regional channels and more
than 2000 Class 3 stations. These numbers were taken before expansion of
the AM broadcast band which has grown the totals.
Class 4 stations operate in assigned local channels with no more than
1kW day and 250W night assignments. There are 6 local channels with 150
or more Class 4 stations on each channel.

Primary service area is the statiob`s groundwave coverage. Secondary
service is uninterfered skywave coverage. Intermittent service lies
between the primary and secondary service areas.

A clear channel has one or more high-powered stations which serve wide
areas. All primary service areas and a substantial portion of their
secondary service areas are cleared of objectional interference.

A regional channel has stations not exceeding 5kW which have coverage
contours which limit the primary service interference between these
stations.

A local channel has stations not exceeding 1 kW daytime and 250 watts at
night. Primary coverage is limited by interference. Assignments are made
to limit interference.

Radio waves are radiated into a hemisphere as space below the antenna is
hidden by the surface of the earth. This results in a formula for the
field power at one mile from a perfect radiator emitting 1 kilowatt:
P = 1000 / 16266419 = 0.00006 watts/sq mtr
E = sq rt (PR)
and R=377 ohms

Volts/mtr=152 at 1 mile from a perfect infinitely short uniform
hemispherical radiator.

From a 1/4-wave vertical, it`s about 195 millivolts per mtr at 1 mile.

From a 1/2-wave vertical, it`s about 236 millivolts per mtr at 1 mile.

From a 5/8-wave vertical, it`s about 267 millivolts per mtr at 1 mile.

Volts vary as the square root of the power. So, for 50 killowatts,
multiply the 1 kilowatt values by 7.07.

The field strengths are the inverse distance or lossless values. Real
earth has losses.

Best regards, Richard Harrison, KB5WZI

Richard Fry wrote:
"The most common radiator height for Class A non-directional AM
broadcast stations operating at 50kW day and night is 195-degrees."

"Richard Harrison" responded (in part)
I won`t challenge that as I have conducted no survey. WJR Detroit is
shown on the broadcast allocations map book as an unlimited (day and
night) 50 kilowatt Class 1 station.
__________________

Your text is referenced to out-of-date versions of applicable FCC Rules.
The FCC adopted the metric standard over 15 years ago, and the
classification of AM broadcast stations no longer is defined as Class 1 to 4
but Classes A, B, C & D. The current versions of the applicable Rules are
contained in 47CFR Part 73, and for this topic are dated October 1, 2004.

Minimum radiator heights in meters for Class A, B and C AM broadcast
stations are shown in Figure 7 of 47CFR73.190. Radiator efficiency for
Class A stations (other than in Alaska) must be such as to produce a ground
wave of least 362 mV/m at 1km for 1kW of antenna input power. A 90° omni
radiator cannot do that.

RF