Hidden Damage to 450-ohm Balanced Line

Recently I installed new coax cable from my rig to the outdoor surge protector on the back of my garage, where the balanced feed line terminates from my multi-band dipole. This is the latest evolution of the dipole from its initial design based on pages 7 - 10 of the article Choosing the Correct Balun.  The current configuration has adopted some characteristics of a G5RV design.

While handling the 450-ohm window line I noticed that a short (1.25") section of one side of the line was able to bend at a much sharper angle than other parts of the line. Since this was part of the length I cut off, I examined it more closely by slicing away some of the insulation. The photo below shows what I saw.

Click to enlarge

It appeared that one strand of the stranded wire was still connected. I can't tell if this localized damage relates to the lightning strike in 2016 or the recent lightning strike to a nearby tree or high-power operation on 40 m or a combination of these events.

I am continuing to use and modify this antenna.

73,
John

Multi-band Dipole - version 2

This posting continues my exploration of the performance of my multi-band dipole as reported earlier. Based on suggestions from KE4ID, I revised my antenna to the following specs:

• 102 ft total length of horizontal portion (previously 110 ft)
• 91 ft of 450-ohm ladder line (previously 84 ft)
• DX Engineering 1:1 balun (also tested with LDG 1:1 balun on 80 m band)
• 12 ft of 50-ohm coax to the antenna tuner

Update: Ultimately, I may need to change the feed line to 300-ohm ladder line. The current configuration is an attempt to reuse the 450-ohm line that fed my original 80 m loop at this location. Due to the loss of one of my corner support trees, I reconfigured the antenna materials to a dipole. The recommended feed line for a multi-band shortened dipole, used with a tuner, is 300-ohm ladder line.

The graph below shows the SWR vs frequency as measured with my antenna analyzer across the range of 3 to 30 MHz. The wide variation of SWR is an indication of the complexity of the overall antenna system. Also shown (as discrete points) are SWR data for selected frequencies on the transceiver side of the antenna tuner. These points show that the tuner is able to present reasonable SWR to the transceiver on most of the frequencies I sampled. An area of challenge is the 3.5 to 4 MHz band. The next graph shows this frequency range more closely.

SWR - 3 to 30 MHz

The graph below shows the frequency range of 3.5 to 4 MHz. Along with the same SWR data measured by the analyzer there are two sets of SWR points from the transceiver side of the antenna tuner. Testing with the LDG 1:1 balun provided better results than the DX Engineering 1:1 balun at the lower end of the frequency band. Similar high SWR results at the low end of the band were also obtained with the previous configuration of the antenna and feed line. 

SWR - 3.5 to 4 MHz

The power efficiency of this antenna system (as built) is unknown. Instrumentation on the antenna side of the balun might reveal how much power or current is outbound to the antenna, and how this design compares with resonant antenna designs.

While adjusting the overall length of the antenna from 110 ft to 102 ft, I had the opportunity to notice lightning-caused damage from last summer's event reported here. The two photos below show the effects at the western end of the antenna. Click on each image for the full size view.

Lightning damage to antenna

On July 14 my multi-band dipole antenna had a lightning encounter of some degree. Below are some photos showing damage to the feed point and the surge arrestor that is mounted on the outside wall of the garage.

 Here is a view of the feed point after I lowered it to eye level. The front part of the Ladder-Loc mount was blown away, but the electrical connections are intact.

Later I found two pieces of the front part and put them in place as shown in this photo. I have not found the other pieces.
The front part of a new unit might fit in place of the one that was damaged, if the dimensions have not changed much. I could simply secure the feed line and use the back part as is, without a front.

 This photo shows the inside of the balanced line surge arrestor, model AS-309H from Array Solutions. The referenced PDF file gives specs and explains the design features.
The feed from the antenna enters at the bottom, connected to the two long machine screws. At the top end of each screw a gas tube protector and a 1 Mohm resistor connect to a ground lug. The resistor provides a path for static charge to bleed off. A capacitor at the top of each screw connects to the balanced output lugs for the portion of feedline that enters the house.

Here is an annotated close-up of the area that shows the visible damage. Click on the photo for a better view. The two resistors have normal readings. I plan to replace both gas discharge tubes. The two capacitors are not shorted, based on ohmmeter readings, and don't appear to be open, based on transceiver operation and SWR readings. They may have changed somewhat in capacitance and should probably be replaced, too.

At the transceiver end of the feedline, I inspected the balun and found no visible damage. Ohmmeter readings revealed no shorts or grounds.







Update: In February of 2017, when adjusting the lengths of the dipole legs, I found additional evidence of the lightning hit at the southwest end of the dipole. See the two photos at the end of this posting.

John

Frequent Lightning Disrupting 75 m

Frequent lightning on the morning of 4/27/16 dominated my reception on 3740 kHz (and all of the 75 m band). The 24-hour map of lightning below shows where the activity was located. Yellow represents data that is up to 15 minutes old, relative to 0535 CDT. Orange is up to 2 hours old, and red through blue represent data that is 2 to 24 hours old.

From LightningMaps.org - Click to enlarge.

My SDR receiver display below shows that static crashes were very frequent and very broad-banded.The top part of the display is the panadapter format, with signal level represented on the vertical scale and frequency (kHz) on the horizontal scale. The bottom part of the display is the waterfall format, with time progressing vertically down the display, frequency on the horizontal scale, and average signal level represented by color.

Panafall display at 0623 CDT - click to enlarge

Compare this display with the one in my earlier post that showed less frequent lightning activity compared with a nice strong signal from W4PRE.

Weather Sites

The web offers us many richly-detailed web sites devoted to weather. Here is a list of many of the best known, and a few others, in no particular order:

• National Weather Service
• Storm Prediction Center
• The Weather Channel
• WeatherSpark
• Intellicast
• Weather Underground
• AccuWeather
• WeatherBug
• Alabama Weather Blog

Lightning data maps:

• LightningMaps
• StrikeStar
• Vaisala
• Blitzortung

Tuesday night squall line

Here are two screen shots I saved as the front with squall line arrived in Baton Rouge.

Click above to enlarge.

 2-hour lightning history from LightningMaps.org, as of 2248 CST. 

Click above to enlarge.

Weather radar from Weather Underground Wundermap, as of 2254 CST. The red place marker is Baton Rouge.

Lightning Maps

During this morning's QSO I disconnected the antenna feed twice due to nearby lightning. I looked at various lightning data web sites I had bookmarked and found one that let me pan and zoom to the degree I wanted. Below are two screen shots from LightningMaps.org that uses data from Blitzortung.org to overlay a Google map. When I selected Real Time near the upper left corner on the initial map I found I was able zoom in and pan. I also could select the amount of history to display, using check-boxes across the top of the map.

First is the view captured at 0722 CDT, 3/10/2015, with 2 hours of data. 

Click above to enlarge view.

Next is the view from 0930 CDT and shows 24 hours worth of data, using more colors.

Click above to enlarge view.

I intend to keep this site handy for future reference.

Weather Radar Websites

Per discussion on 3740 kHz on a recent morning, here are a few of my favorite websites for weather radar information:

• National Weather Service - Southern Mississippi Valley Sector
  This one provides a nice quick overview of my region to see what is in nearby states. Other sectors can be selected, along with a full national view. You can click on the word Loop to animate any sector view over a 70-minute interval. By clicking on any city you will be taken to the nearest local radar view, such as this view for Mobile. Various controls are available there, and radar animation can be activated for a 40-minute loop. The animation requires JAVA to load since it is a JAVA applet. Since the NWS is funded by us taxpayers, there are no ads.
• Weather Underground's WunderMap - To reach your local view go to the home page, enter your zip code in the Search box near top of page and click Go. On the resulting page you will find lots of information and map displays. Below the little radar thumbnail you'll find links for Local Radar, Hi Def Radar, WunderMap, etc. Click on the WunderMap link. The resulting display shows radar information and Weather station markers that you can click for local conditions at those sites. You can un-check the Weather stations box in the Map Controls section under the map if you want to simplify the display. There you can also find a Start link for animating the radar display. Additional options are available under Radar Options. You can 
• zoom in or out easily using the + and - buttons on the upper left side of the radar map
• pan using the arrow buttons or by click and drag on the map
• select the Map, Satellite, Hybrid, or Terrain view
• StrikeStar lightning map
  This site displays lightning activity over the past 60 minutes or a 24-hour summary across the USA. Regional views (e.g. Southeast) can be selected to show a better view close to your current location.  Color coding of the data display is defined near the upper right side of the map.