Thanks Walt!

"walt" wrote in message
...
On Aug 23, 8:38 pm, Gary wrote:
On Aug 23, 4:53 pm, "Peter O. Brackett"
wrote:



Roy:

Thanks for your input on the effects of interference from other
transmitters, especially BC transmitters, point well taken.

I suppose one might add some kind of carefully designed passive
filtering to
the devices to 'notch' out offending BC stations, etc...

But... that would be messy and complicated.

Using a bridge with a tuned detector seems a much better approach. I do
have an old MFJ Rx resistance bridge at hand, but I was looking for a
little
bit better accuracy. I must check e-bay, etc... for prices for a used
bridge of the GR class you have.

My current problem could likly be solved by using one of my (so-called)
VSWR
meters, but I felt I might like to get a little more metrological
capability
for a few bucks.

I've never had a vertical antenna before and currently I'm laying out a
radial field under a new vertical antenna. I just wanted to know when to
stop laying down radials.

I figure that I just need to measure the input impedance of the antenna
Zin
at my frequencies of interest and record the (hopefully decreasing)
impedance values as I lay down more radials. My assumption is that Zin =
Zant + Rg. where Zant includes the reactance and radiation resistance of
the radiator element at my frequency of interest and Rg is the ground
resistance. Hopefully Rg should decrease as I add raials. I intend to
quit
adding radials when the impedance decrease becomes 'negligible' ( In the
sense of received S units [smile]).

Thanks again!

-- Pete k1po
-- Indialantic By-the-Sea, FL

"Roy Lewallen" wrote in message

...

I have only one comment which applies to most if not all of the amateur
level products, as well as a very high quality HP impedance meter I
used
years ago.

I live about 15 miles from hilltops where the local AM, FM, and TV
broadcast towers are. The signals from those stations are strong
enough to
overwhelm the broad bandwidth detection circuitry in the analyzers
(and HP
vector impedance meter) I've used, resulting in meaningless readings
when
connected to an antenna. I find that I grab my MFJ antenna analyzer
for a
host of jobs like measuring a ferrite core impedance, checking the
length
of a piece of coax, and so forth. But for me it's just about useless
for
the job of analyzing actual antennas. When I need to measure antenna
impedance I dust off an old GR bridge and use a portable receiver for
the
detector in order to reject the strong ambient signals.

Of course this isn't a problem for everyone, but it sure is for me and
I
don't think my situation is unusual for an urban environment. I've had
to
put common mode chokes on my thermometer thermocouple wire, my light
meter
connecting wire, scope leads, and even in audio circuitry to keep the
RF
out. But even one strong local station might be enough to upset a
typical
antenna analyzer.

Roy Lewallen, W7EL

On 8/23/2010 11:15 AM, Peter O. Brackett wrote:
Antenna Newsgroup Denizens:

Apart from the professional 'lab grade' (and undoubtedly very
expensive)
devices made by the likes of Agilent, there are on the amateur market
several antenna analyzers that are more reasonably priced.

I'm currently considering the purchase of such a device, and so...

I'm interested in hearing opinions, pro-con arguments, and/or
receiving
pointers to reviews of such devices.

All thoughts and comments will be appreciated, unbiased or not.

Thanks!

-- Pete k1po
-- Indialantic By-the-Sea, FL- Hide quoted text -

- Show quoted text -

Hi Pete, If you are evaluating a radial field for a vertical, then
the value you should be shooting for is 30-35 ohms, with no
reactance. As you know this is 1/2 the impedance of a 1/2 wave
dipole-70 ohms in free space. I have never done this, but an
impedance below 40 ohms with little reactance should provide you with
an efficient vertical. Once you get around this value, I guess there
is nothing to be gained by adding more radials. What your analyzer
tells you, as others have mentioned, depends on the rf in the area.

Gary N4AST

Hi Pete, long time since I've seen a post from you on this NG!

Pete, I'd take Roy's route and use a GR impedance bridge. I've used
the GR-1606A and the 1606B for the last 50 years. If you're not
familiar with it it's been the standard impedance-measuring device for
AM BC antennas for more than 50 years. It's accuracy cannot be beaten.
These bridges were expensive when new, but they are available rather
inexpensively now, and are stable as a rock. And as Roy said, they use
either a millivolt meter or a tunable receiver as the detector. Using
the receiver, interfering signals picked up with the antenna being
measured are eliminated, thus not degrading the accuracy of the
impedance measurement.

The GR-1606A measures from below 500 kHz up to 60 MHz. The GR-1602
measures well into the VHF range. If you should try one I know you'll
like it.

Walt, W2DU

PS--If you'd like to see the results of measuring W2DU's antenna
impedances, I refer you to Reflections, Chapter 15, Tables 15-4, 15-5,
and Fig 15-1, which is a graph of the data in the Tables. If you don't
have a copy of Reflections you can see this data on my web page at
www.w2du.com. Just select 'Read Chapters from Reflections 2'.