[TowerTalk] "Magic" length from tower to first insulator

[StellarCAT]

not to anyone specifically in this list of replies ... 

I had ~6’ of steel from the tower to the first insulator and many insulators after that using the ARRL AB values. I had 3 sets of guys – 40’, 80, 120. There was a C31 above each of the guys – 1’ up. The bottom one saw the worst interaction relative to SWR (and again: per lots of modeling the SWR is the second most sensitive preceded by the FB). there was a noticeable effect as I rotated the antenna (tower). 20 not too bad but there on the bottom one – 15 more so – 10 a good deal so. The middle guy was not as bad but still there ... the top one was almost without issue (its angle being pretty sharp to the antenna). 

My suspicion is there is some other conjunction going on between all three guys starred out from and through the tower not taken into account. 

I solved it by attaching some generic ~3/8” diameter (internal) clamp on ferrites. 3 (4 on the bottom guy) to each line mounted literally at the end of the big grip thus making that first ‘disconnect’ at around 30” out or so... this for all practical purposes solved the issue. The idea was that of AB7E. 

Since then I had thought about doing this again on 2 towers I’m planning here. 147’ rotating and a 110’ fixed... but modeling the guys I find that it takes a pretty substantial resistive component to the ferrites to get them to effect enough change. And I’ve not been able to find one that would be suitable. A 31 core is the closest. Also note that currents get set up all over the place – guys above – guys below – guys near the end more than 100’ away ... so there IS a substantial amount of interaction mostly seen in FB changes (mostly degraded – sometimes improved). My gut says that those that say its not that bad are speaking anecdotally. Indeed if you’re really not worried – then I’m sure its fine ... for me – wanting to put up the best I can for my investment in time and money – its not good enough. I finally decided to go with 50% philly coming off the tower and the rest EHS. I’ll have ~10’ of EHS off the tower and I’ll do multiple ferrites to break it up at the tower and 5’ out so I BELIEVE it will be two 5’ lengths and not connected to the tower and thus of no net effect. 

Gary 
K9RX


From: StellarCAT 
Sent: Wednesday, March 16, 2016 7:50 AM
To: tower 
Subject: "Magic" length from tower to first insulator

I used the ARRL AB values and found that the distance recommended from the tower out was problematic. I had quite severe interaction to the guys with only about 6’ from the tower to the first insulator. My guess is that they weren’t taking into account that it is connected to the tower at the center and there are 3 of these 6’ lines going out ... the impact was seen in the SWR so as noted below the FB would have been severely impacted. This was solved by placing clamp on ferrites just at the end of the Big Grips at the tower (3 on each leg). 

As for divorcing the first section and not having any interaction beyond that – not a chance! I’ve modeled guy lines with yagis and they interact considerably. The first hit is to the FB ratio where it can reduce it by 5+ db (it can also enhance it in some directions)... the second hit is the SWR ... the gain is rarely impacted. 

I personally am going with 50% philly and then EHS. This works very well in eliminating interaction at least in the models I’ve done for the bands I’ve done. 

Gary 
K9RX


A friend of mine told me that someone, somewhere had calculated a "magic"length for guy cable. If you went down 10 or 11 or 13 feet (or whatever itis) off of your tower and installed your first insulator there, your guyswould be sufficiently 'non-resonant' so that they wouldn't interfere withthe pattern of the antenna(s) above.What is this magic length and where is it documented? Larry WO7R_______________________________________________