"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:srSpi.4065$9A6.827@trnddc01:


"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...
You might try the matching system used on the old "Gotham
Vertical". This uses a tapped coil from the base of the mast to
ground. The top of the coil is tapped down until resonance is
found. Coax braid is to ground, and the coax center conductor is
tapped up from the bottom of the coil for best match.

Whilst this system might work for an antenna with a capacitive
feedpoint, will it work for an inductive feedpoint such as this ~3/8
lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't
that work?

I don't really understand exactly what you mean, though I know it is a
popular proposition.

If the antenna feedpoint is around 100+j300 (see my previous posting),
the simplest matching network is an L network, and there are two
solutions to it, but both L networks in this specific case require an L
and a C, and both use practical values of L and C. For example, a 84pf
capacitor in shunt with the feedpoint, then a 2.4uH inductor in series
from the feedline to the feedpoint will deliver that transformation with
reasonable efficiency and with one side of the capacitor grounded.

Can you propose the values of a two inductor L network, or a tapped
inductor that will transform 100+j300 to 50+j0.

Owen
I spent some time pondering this with EZNEC and a smith chart. No, I can't
come up with an inductive way to match it. I learned somthing
today...thanks!

On Jul 26, 11:55 am, "Wayne" wrote:


I spent some time pondering this with EZNEC and a smith chart. No, I can't
come up with an inductive way to match it. I learned somthing
today...thanks!

A 3/8 wave radiator is easily matched with a single variable cap..
Or should be anyway...
MK

wrote in
ups.com:

On Jul 26, 11:55 am, "Wayne" wrote:


I spent some time pondering this with EZNEC and a smith chart. No, I
can't come up with an inductive way to match it. I learned somthing
today...thanks!

A 3/8 wave radiator is easily matched with a single variable cap..

Details?

Or should be anyway...
MK

"Wayne" wrote in
news:Ll4qi.15820$U47.4532@trnddc08:

....
Owen
I spent some time pondering this with EZNEC and a smith chart. No, I
can't come up with an inductive way to match it. I learned somthing
today...thanks!

The components for a L matching network may both be L for some range of
loads, but not in the case discussed.

There will also be loads where only a shunt component is required, and
this is often achieved on a short vertical by using less inductive
loading than needed to bring the feedpoint to a purely resistive load.
You can think of it as an L network where the series element is supplied
by the antenna. For example, a short antenna on 3.5MHz with a feedpoint
resistance of say 20 ohms, and a large capacitive reactance can be
inductively loaded to deliver 20-j24.5, and it will require only a shunt
inductor of about 0.9uH to match it to 50 ohms.

(Neither of these are solutions to the original questions, but
interesting sidepaths that show why some configurations work in some
situations.)

Owen

On Jul 26, 5:14 pm, Owen Duffy wrote:
wrote roups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?


Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK

wrote:
On Jul 26, 5:14 pm, Owen Duffy wrote:

wrote roups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?



Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK



G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax is
OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in this
case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.

cliff wright wrote in
:

wrote:
On Jul 26, 5:14 pm, Owen Duffy wrote:

wrote
legroups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?



Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK



G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.


Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the
limitations to his method.

The proposed series capacitor will only achieve a reasonable match if the
series resistance component of the feedpoint is around 50 ohms, and the
reactance is inductive and within range of a practical capacitor.

If indeed the antenna had a feedpoint impedance around 100+j300, the
series capacitor would not achieve a match better than VSWR~=2 (which may
or may not be good enough). Your observation that it was bad (whatever
"quite a high VSWR" means), suggests that it has a higher feedpoint R (it
is unlikely to be lower than 50 ohms), or that your capacitor did not
tune out the inductive reactance (unlikely since you used a 500pF
variable).

If you can't match it with a series cap (and no surprises for me there),
you could always try an L network starting with the values that I
previously gave you.

Owen

Owen Duffy wrote:
cliff wright wrote in
:


wrote:

On Jul 26, 5:14 pm, Owen Duffy wrote:


wrote
glegroups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?



Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK




G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.



Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the
limitations to his method.

The proposed series capacitor will only achieve a reasonable match if the
series resistance component of the feedpoint is around 50 ohms, and the
reactance is inductive and within range of a practical capacitor.

If indeed the antenna had a feedpoint impedance around 100+j300, the
series capacitor would not achieve a match better than VSWR~=2 (which may
or may not be good enough). Your observation that it was bad (whatever
"quite a high VSWR" means), suggests that it has a higher feedpoint R (it
is unlikely to be lower than 50 ohms), or that your capacitor did not
tune out the inductive reactance (unlikely since you used a 500pF
variable).

If you can't match it with a series cap (and no surprises for me there),
you could always try an L network starting with the values that I
previously gave you.

Owen

Yes Owen I reckon you are right!
Tried it out again today and the VSWR is a bit worse than 2:1, not good
enough for me at all.
Now all I need is a good reference work to brush up my complex
impeadnaces that I haven't looked at for about 30 years!
73's ZL1BDA

cliff wright wrote:
Owen Duffy wrote:

cliff wright wrote in
:

wrote:

On Jul 26, 5:14 pm, Owen Duffy wrote:


wrote
roups.com:



A 3/8 wave radiator is easily matched with a single variable cap..


Details?




Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK




G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.



Well, it isn't quite "viola" or even "voila!" is it. MK didn't state
the limitations to his method.

The proposed series capacitor will only achieve a reasonable match if
the series resistance component of the feedpoint is around 50 ohms,
and the reactance is inductive and within range of a practical capacitor.

If indeed the antenna had a feedpoint impedance around 100+j300, the
series capacitor would not achieve a match better than VSWR~=2 (which
may or may not be good enough). Your observation that it was bad
(whatever "quite a high VSWR" means), suggests that it has a higher
feedpoint R (it is unlikely to be lower than 50 ohms), or that your
capacitor did not tune out the inductive reactance (unlikely since you
used a 500pF variable).

If you can't match it with a series cap (and no surprises for me
there), you could always try an L network starting with the values
that I previously gave you.

Owen

Yes Owen I reckon you are right!
Tried it out again today and the VSWR is a bit worse than 2:1, not good
enough for me at all.
Now all I need is a good reference work to brush up my complex
impeadnaces that I haven't looked at for about 30 years!
73's ZL1BDA

Owen I just happened across a reference on page 248 of "Amateur radio
Techniques" by Pat Hawker which I had missed before.
LUckily I have a good stock of "minductor" type coils and hope sometime
next week to try the arrangement at(d) in figure 64.
I have also got a stock of miniductor tapping clamps so it will be easy
to vary the feed point and tuning connections.
With a bit of luck, and some good weather I might have it all set up by
about Friday next.
Many thanks for your suggestions.
73"s Cliff Wright ZL1BDA ex G3NIA.

On Jul 27, 8:21 pm, Owen Duffy wrote:
cliff wright wrote :



wrote:
On Jul 26, 5:14 pm, Owen Duffy wrote:

wrote
legroups.com:

A 3/8 wave radiator is easily matched with a single variable cap..

Details?

Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK

G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.

Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the
limitations to his method.

I wasn't really expecting the feed R to be that high.. If it's 100
ohms,
then yep, the best match would be 2:1. If that's the case, then yes,
he would need to use an L network if he wants the match better than
that.
I never had any trouble using this method, but thinking about it, most
of mine were with 160m inv L's.. Maybe it was the shorter vertical
section that gave me a low enough feed Z to match .
I'd have to model it to see.
MK