Charlie Tango 11m DX Radio Group Forum
Would you like to react to this message? Create an account in a few clicks or log in to continue.


Welcome to the Charlie Tango DX Group. The home of ALL things Radio - CB, 11m DX, Amateur, PMR446, PLD, Network and Data. We welcome all radio enthusiasts of all knowledge levels. Join today and claim your own unique World Famous CT Call-sign
 
HomeEventsCalendarFAQSearchRegisterLog in
Have You Just Arrived From Old Forum? : If you have then you need to click "Join CT" and register an account on this new forum. Your CT call sign is still valid and you don't need to apply for a new one.
CT Summer Prize Draw: The first number drawn will win a Brand New President Mckinley Tuned Export 11m all mode radio!! See the Latest News forum for details.
Runner-Up Prize: The second number to be drawn will win a £30 Paypal Gift. There is still time to get involved.
Latest Buxton 2020 News: Unfortunately, due to the Covid-19 pandemic we have had to cancel this year's Buxton date. Sorry for any inconvenience.
Navigation
Useful Radio Sites
Log in
Username:
Password:
Log in automatically: 
:: I forgot my password
CT Bill Board





Views This Year
All Time Views
Classified Adverts

Latest topics
» Rotel 240 power
Antenna installation (home base)  Icon_minitimeToday at 8:19 pm by glenn dog

» Win A President McKinley or £30 Paypal Deposit - CT 2020 Summer Prize Draw
Antenna installation (home base)  Icon_minitimeToday at 8:08 pm by Firetrap

» channel display on Baofeng uv-5r
Antenna installation (home base)  Icon_minitimeToday at 7:48 pm by dalesman

» 2020 Prize Draw Tickets and Entry
Antenna installation (home base)  Icon_minitimeToday at 7:40 pm by Firetrap

» Hi everyone
Antenna installation (home base)  Icon_minitimeToday at 7:30 pm by Tex853

» National Hamfest canceled
Antenna installation (home base)  Icon_minitimeToday at 6:50 pm by STARSHIP

» can i have call-sign please
Antenna installation (home base)  Icon_minitimeToday at 5:49 pm by munz

» Buxton Bash 2021
Antenna installation (home base)  Icon_minitimeToday at 5:40 pm by Paddy

» What Balun/Unun should I use?
Antenna installation (home base)  Icon_minitimeToday at 5:32 pm by Templar

Keywords
frequency Calling chart tango division callsign call 6900n ss9900 9900 alan president lincoln tickets yaesu list power antenna radio ALINCO Buxton Mckinley Mobile Sirio midland sign
QRZ Database
 

 Antenna installation (home base)

Go down 
AuthorMessage
Patwah81
New Member
New Member
Patwah81

Call Sign : 26-CT-3353
Posts : 5
Times Thanked : 0
Join date : 2020-02-10
QTH or Location : Nottingham
Equipment Used : President Grant 2. Thunderpole Half Wave. UV-5R. Inrico T320

Antenna installation (home base)  Empty
PostSubject: Antenna installation (home base)    Antenna installation (home base)  Icon_minitimeSat Mar 07, 2020 9:23 pm

Hi all,

I will be installing my new setup next weekend (hopefully) and I read online that the length of the coax from the antenna to rig should be 18ft. The antenna I will be installing is a Thunderpoll half wave that is also 18ft (it comes pre marked to 11m band) Do I still need the 18ft of coax in between? The coax I have is mini8
Any help will be greatly appreciated.

best 73  to all.
Back to top Go down
View user profile
Northern Crusader
Contributor
Contributor


Call Sign : M0GVZ / 26CT1760
Posts : 244
Times Thanked : 12
Join date : 2019-11-13
QTH or Location : IO94SA
Equipment Used : Icom 7300, TS480, President McKinley, Albrecht AE6110, CRT Mike
Age : 49

Antenna installation (home base)  Empty
PostSubject: Re: Antenna installation (home base)    Antenna installation (home base)  Icon_minitimeSat Mar 07, 2020 10:57 pm

The people who say it should be 9ft, 18ft or whatever don't know what they're talking about. The correct length is the length needed to get from radio to antenna by whatever route you need to.

If you want a technical explanation as to why the old wives tale that coax needed to be a certain length then I'm happy to explain why people thought it needed to be but long story short it has everything to do with a poor or non-existent RF ground and as a result the length of coax having an effect on the SWR which, if you have a decent RF ground, it shouldn't.
Back to top Go down
View user profile
GaryWilson
CT Directors
CT Directors
GaryWilson

Call Sign : 26-CT-1836 Ham 2E0GGQ
Posts : 1484
Times Thanked : 121
Join date : 2019-06-27
QTH or Location : London
Equipment Used : Radios & Wires & Stuff!
Age : 55

Antenna installation (home base)  Empty
PostSubject: Re: Antenna installation (home base)    Antenna installation (home base)  Icon_minitimeSun Mar 08, 2020 7:18 am

Exactly what Connor has just told you Peter. Very Happy  

73's Gary.
Back to top Go down
View user profile https://www.qrz.com/db/2E0GGQ
stephen Gunrunner
New Member
New Member
stephen Gunrunner

Call Sign : 26CT526/MR021/M6XXX26DD666.
Posts : 75
Times Thanked : 1
Join date : 2019-06-27
QTH or Location : wooler north northumberland or some were near it
Equipment Used : ss9900 mass dx 5000 sirio 4000hp sirio 5000 turbo trimag tti 801 crt7u
Age : 63

Antenna installation (home base)  Empty
PostSubject: Re: Antenna installation (home base)    Antenna installation (home base)  Icon_minitimeSun Mar 08, 2020 11:18 am

and if you can get the best coax you can afford, my antenna is about 8m from the house and about 14ft to the base of the antenna, and I used the best I can afford but right now my antenna is busted swr when put up 1.2 1.3 but after last big storm swr is into the red so nothing to see out side of the antenna joints well taped and nothing has moved, but a slight bend so I think its in the base of the antenna so a new A99 will be going up at the end of the month. not the best antenna but I am to old to be putting up and taking down, aluminium antennas never last long up here
Back to top Go down
View user profile
Patwah81
New Member
New Member
Patwah81

Call Sign : 26-CT-3353
Posts : 5
Times Thanked : 0
Join date : 2020-02-10
QTH or Location : Nottingham
Equipment Used : President Grant 2. Thunderpole Half Wave. UV-5R. Inrico T320

Antenna installation (home base)  Empty
PostSubject: Re: Antenna installation (home base)    Antenna installation (home base)  Icon_minitimeSun Mar 08, 2020 12:05 pm

Thank you guys, the help is much appreciated. If you have the time for a more technical reason to the wives tale that would be great, but if not that's ok too.

As for the coax, I could only really afford the cheaper stuff (RG mini Cool. Someone recommended Aircell 7 but I think that will be a fair way down the line before I can upgrade to that kind of coax, or maybe you could recommend a type of coax.

Once again, thank you for your help Smile
Back to top Go down
View user profile
Northern Crusader
Contributor
Contributor


Call Sign : M0GVZ / 26CT1760
Posts : 244
Times Thanked : 12
Join date : 2019-11-13
QTH or Location : IO94SA
Equipment Used : Icom 7300, TS480, President McKinley, Albrecht AE6110, CRT Mike
Age : 49

Antenna installation (home base)  Empty
PostSubject: Re: Antenna installation (home base)    Antenna installation (home base)  Icon_minitimeSun Mar 08, 2020 2:02 pm

Patwah81 wrote:
Thank you guys, the help is much appreciated. If you have the time for a more technical reason to the wives tale that would be great, but if not that's ok too.

First of all you have to look at an antenna as being a complete system. You have the bit that sticks in the air which you want the signal to go out of, you have the bit it mounts to and the bit that sends a signal to the antenna. Everything in the system affects the tuning of the antenna (and therefore SWR), some of it which is intentional, some of it which happens because of an issue with the installation.

The physics behind it is a bit complicated but basically it's all about the flow of electrons and Kirchoffs Law (which I'll let you read up about), in particular his first law.

For this next bit I'll quote from W8JI who explained it simply much better than I can.
Quote :

When we force charges up into an end fed antenna (making current flow), we have to move an equal number of charges out of some ground system or counterpoise at the feed point. We always must have the same current coming into the feed point from some sort of counterpoise as the current that moves up into the antenna at that point! There is no way around that rule.

Another way to view this is feed lines or feed points of our end fed antennas must have something to push against to force current into the antenna. It is very much like pushing a car. If we have very poor footing, our feet will move and slide as we push. We not only waste energy that could be used to move the car, we have movement or motion where we don't want movement. The same is true for a ground system, as feed line power "forces" current up into the antenna the other terminal of the feed line has to be held steady. We waste energy that COULD be in the antenna, and we have RF movement where we don't want it....on the outside of the coax shield and on anything connected to that shield.

So reading the above you need a RF ground and normally to do this correctly we use radials which could be made from wire, ally tube, whatever.

When we don't put in an adequate RF ground then the antenna system will try to compensate for it. It'll use anything it can source electrons/current from to compensate. This inevitably means it'll use the mast it's mounted on if there'a an electrical connection and any wire connected to it which means your coax which we don't want.

When it's using the mast and, more importantly, the coax we have RF spewing out everywhere we don't want it to and not that much going out of the antenna. That's caused by what is known as common mode RFI which involves RF travelling on the outside of the braid of the coax and it means you pick up more noise, you're more likely to cause interference to things like TVs, burglar alarms etc, and if it's bad enough it'll distort your transmit audio and if you're running power it can lead to RF burns. Some people have reported a tingling sensation from the mike when transmitting, that's actually RF burning caused by common mode RFI and the RF ground system being so poor for the power being transmitted that the antenna system is using the coax AND the radio and the power cables and the mike cable to try to compensate.

So now how the old wives tale came into being. A lot of CBers noticed that when they cut their coax to certain lengths that the SWR dropped and that it was lowest at multiples of 9ft or thereabouts. Now remember how I said at the start that the antenna works as a system. What they didn't realise is the reason why is that they had a poor or non-existent RF ground so the antenna system was using the coax as a RF ground. By altering the length of that coax, because it was forming part of the antenna (which we don't want) then it was altering the tuning of the antenna. Lo and behold, the old wives tale is born. What they didn't know was that the reason it did was because of an actual problem, that if the SWR alters because you change the length of the coax that the antenna system is telling you you don't have an adequate RF ground at the antenna.

So how do we avoid the coax becoming part of the antenna? A decent RF ground and using a RF choke at the antenna end of the coax. You can either build or buy one using a ferrite core or you can make it from the coax, winding 5 turns around a 4.25" diameter former (110mm plastic ground waste pipe you can buy from MKM is ideal). That'll present a high impedance to RF at 27MHz, preventing or seriously reducing the ability for RF to travel down the outside of the coax and preventing common mode RFI.
Back to top Go down
View user profile
 
Antenna installation (home base)
Back to top 
Page 1 of 1

Permissions in this forum:You cannot reply to topics in this forum
Charlie Tango 11m DX Radio Group Forum :: Community Hangout :: General Chat-
Jump to: