I downloaded the demo of EZNEC4 to see what it would show for a 88ft
doublet 50ft in the air fed by 300ohm twinlead.

My model has two wires each 44ft long joined in the middle at 50ft
elevation with another wire pair 0.1ft long 0.1ft of elevation.

They are joined by two 300ohm transmission lines and a split current
source is placed at the 0.1 elevation junction.

When I run the SWR plot it shows over 100:1 with minima near 2:1 at
6.5Mhz and 11.75Mhz.

Does this sound about right or am I not modeling it correctly?

tnx
jtm

Jim Miller wrote:
They are joined by two 300ohm transmission lines and a split current
source is placed at the 0.1 elevation junction.

You don't need two transmission lines for balanced
line. The dipole can be just one wire with the
single 300 ohm transmission line attached at the
50% point.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Jim Miller wrote:
I downloaded the demo of EZNEC4 to see what it would show for a 88ft
doublet 50ft in the air fed by 300ohm twinlead.

When I run the SWR plot it shows over 100:1 with minima near 2:1 at
6.5Mhz and 11.75Mhz.

Does this sound about right or am I not modeling it correctly?

tnx
jtm

Hi Jim, The 6.5 Mhz SWR sounds about right, the 11.75Mhz does not.
What are you trying to do, an exercise in modeling, or a specific
antenna and transmission line for a specific band?
This is just me, but for complete antenna systems, I use EZNEC for
the antenna input Impedance, and other stuff for the transmission lines
and matching networks.
Gary N4AST

Cecil Moore wrote:
Jim Miller wrote:
They are joined by two 300ohm transmission lines and a split
current
source is placed at the 0.1 elevation junction.

You don't need two transmission lines for balanced
line. The dipole can be just one wire with the
single 300 ohm transmission line attached at the
50% point.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World!
120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption
=----

Hi Cecil, I think he is refering to the way you model a non-radiating
transmission line. It is simply two wires, not two 300 ohm lines
attached to the 50% point.
Gary N4AST

OK, at cecil's suggestion I changed to a single 20 segment wire with a
split I source at 50%. I ran the SWR plot against a 300ohm alternate
impedence. Is that the way to see what effect a 300ohm twinlead will
have? Now I get a minimum 2:1 SWR at 5.6Mhz and a pretty smooth convex
curve out from there to 14.5Mhz with a max SWR along that leg of around
35 at 12.1Mhz.

tnx
jtm

bzzzt...the minimum at 5.6Mhz was about 3:1 not 2:1...

jtm

wrote:
Hi Cecil, I think he is refering to the way you model a non-radiating
transmission line. It is simply two wires, not two 300 ohm lines
attached to the 50% point. Gary N4AST

EZNEC's native transmission lines are non-radiating.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Jim Miller wrote:

OK, at cecil's suggestion I changed to a single 20 segment wire with a
split I source at 50%. I ran the SWR plot against a 300ohm alternate
impedence. Is that the way to see what effect a 300ohm twinlead will
have? Now I get a minimum 2:1 SWR at 5.6Mhz and a pretty smooth convex
curve out from there to 14.5Mhz with a max SWR along that leg of around
35 at 12.1Mhz.

That's more like it and you don't need a split-I source in the
middle of one wire. Split-I sources are used at the intersection
of two wires. 492/88' = 5.6 MHz. 984/88' = 11.2 MHz.

For 7 MHz and up, this antenna would be better behaved with 450
ohm ladder-line or open wire line. The best Z0 for the feedline
is Sqrt(Rmin*Rmax) ~ Sqrt(50*7000) ~ 600 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Jim Miller wrote:

bzzzt...the minimum at 5.6Mhz was about 3:1 not 2:1...

Yep, that puts the feedpoint impedance in the 100 ohm
range. 300/100 = 3
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Hi Cecil

Thanks for your help. Looks like I've got a tiger by the tail. I'm
currently using #26awg fed by 300ohm ladderline to minimize visibility.
Both contribute to pushing the SWR higher across the spectrum. If I'm
not able to get the tuner to match this setup I'll try the 450 ladder
and and 18awg and risk the visibility. Actually I've gotten the antenna
higher than I thought I would be able to and the 18awg would probably
be ok. The ladder line is a bit more difficult to deal with at twice
the width unless I can disguise it or hide it behind a tree trunk. Then
there is still the issue of providing a guard against kids messing with
it.

I'm going to take a multimeter to some white and grey pvc conduit today
to see if I can detect any difference in conductivity as some have
speculated the grey was more conductive. Which ever I chose I'll have
to do some camoflage painting but if the resistance is high and
comparable for both I'll select the smallest diameter which allows the
ladderline to pass through without cramping.

The hardest part of all of this was getting the halyards up and that's
now done. Changing wires isn't a big deal as long as adequate
performance and visibility can be balanced.

tnx
jtm