80 Meter dipole high swr

 I have a 40 meter dipole that I have added a 40 meter trap to and extended the antenna length to allow operation on the 80 meter band.  My previous posts covered the replacement of the traps after the old ones failed due to aging and weather.  I have probably run too much power over the years for the little old traps and helped their failure.

So I replaced them with new, and one of the things to take note of when buying or building a trap like these are what frequency they are resonance at.  Some people have said that just past the lower end (lower in frequency) of the band is the best.  I have not made such claims, as I have not tested that theory.  Advise here is rarely given, but simple observations and measurement.  

The traps I have now are at the top end (higher in frequency) of the band.  

I do not have any preference.

Yesterday, I discovered that on 80 my antenna is resonant down below 3.5 mhz.

So I did some quick measurements, and applying some experience and thoughtful guess work I did a few quick calculations.

Antenna length calculations:

At the start: 468/3.5 = 133.7 feet  

Other end of the band: 468/4 = 117 feet

These lengths do not figure in the loading of the dipole on 80 meters from the traps being inserted.  The traps I have used have always had the effect of shortening the length of the actual dipole.

So the calculated difference in length is 16.7 feet.  This is the difference in length of a dipole on 80 meters that is cut for each end of the bands.  3.5 mhz all the way to 4.0 mhz is very large span to expect an antenna to operate over.

At this point you can divide 16.7 feet / 2 and 8.35 feet of wire needs to come off each end of the dipole.

My goal is not 4 mhz but 3.85 mhz so I need to do some more quess work.

At the start 468/3.5 = 133.7 feet

Target 468/3.85 = 121.55 feet

Difference of 12.16 feet.  / 2 = 6.08 feet to be trimmed off of each end.

I am not brave enough to just cut 6 feet of wire off of the ends, so I will start with 3 feet.  Take new measurements and adapt as needed.  Typically I would never cut 12 feet off of a dipole in the first pass.

Also to note, the height of the antenna is lower then in previous years.  So that also has a effect of lowering the resonance of the dipole.

This antenna has been built and rebuilt several times over the years.  If this was summer time, I would drop both ends and verify that both legs are still the same length.  

Dipole Antenna Rebuild - Spring 2025

 Antenna Repair

After many years of hanging high in the air, my favorite antenna failed over the past winter.  My 40 / 80 meter trapped dipole became too far gone for my antenna tuner to make up the difference. 

Finding the traps in such bad shape, I ran for my MFJ-259B and a few loops of wire to test them.  The oxidation and gunk from nearby trees, gave me a bit of hope, thinking that I might have had a bad connection.  This proved to be false, and the after testing the traps I found that one had gone bad.

Below is my quick and dirty test procedure to check the traps.

This one dipped at 7.26 Mhz and the second one failed.  

Anyone wanting more information, https://history.k4lrg.org/Projects/Repairing_Unadilla_Traps/index.html has some great information on these traps.

I estimate these have been in the air for 15 years.

These PVC traps are now up and have replaced the blue ones. 

These newer traps seem to be at the low end of the 7 Mhz band, where the one good blue one seemed to be at the upper end of the band, using this testing method.  Having these traps out of the antenna, without the rest of the wire antenna connected I expect the measurement to higher in frequency then when in circuit with the rest system.

Also, please enjoy the sun bleached blue jacuzzi cover that I chose to use a work bench... 

As a side note, it seems my wire has stretched by several feet over the years.  While on the roof, I found that I needed to cut about 10 inches off of the 40 meter section.  Whether this was from using different traps or the wire stretching I am not sure.  Stranded wire was used, because it was available and on hand.  

I also see the ends of the antenna that form the 80 meter section was old stranded bare copper wire.  I must have just modified what I had up at the time.  Everywhere else, I use soldered on lug connectors and have rebuilt the antenna as a whole using better methods.  

  

  This is the results of the testing of the replacement traps using my loosely coupled wire loop with the trap just sitting in the middle of the loop.

Hermes Lite 2

 My Hermes-Lite 2

Hermes-Lite information: http://www.hermeslite.com/

Hermes-Lite 2 is an SDR I picked up from https://www.makerfabs.com/search.html?&search=hermes

The groupsio site has a good forum covering the topic of the Hermes-Lite2

Source Code for SerialPortLister on github

Source code and programming info

If you are interested in the source code and programming info for serialportlister, it is posted on github.

 https://github.com/K7UL/Serial-Port-Lister

Software on this site

Aug, 2022

Any and all software posted on my blog for download has been written by myself, unless otherwise noted, and is free to use, give, and share, without asking for my permission.

All other information about my software can be answered by reading my first post.

K7UL

Serial Port Lister

Program: Windows 10 64bit Serial Port Lister

This program lists available serial ports.

This is quicker then opening the settings/control panel and listing devices to find which serial ports are installed.  It stops at COM50.

Download from google drive:  Serial Port Lister

First Post

 First Post

An outpouring of electronic and ham radio information.  No warranty given or implied.  Data and results should be considered with an scrutinizing eye.  Falsehoods may not be apparent, and anything here should be immediately discarded as lies.  Accidental harm may occur.  Intentions expressed here should be ignored.  Explanations will not be given.  No hands will be held.  

Warning issued.

K7UL