I'm always fascinated by the "magic" 9/1 un-un for longwire antennas, also called "Magnetic Longwire Balun". You build this device, connect the transceiver and a longwire antenna of almost random wire, and GO!!! No SWR on 160/10 meters, no Tuners, perfect.....OR NOT?
The background study:
If you google "Magnetic Longwire Balun" you may find a lot of experiences of many ham operators all around the world about this antenna. The cores used for Un-Un are various: ferrite sticks, FT and T Amidon toroids and even cores from old TV EAT transformers.
There is also a SWR table versus wire lenght, found somewhere in the 'net attached to manuals of commercial longwire antennas (click image to enlarge):
I suggest to look at M0UKD web site for a quick guide to build a 9/1, If you want to deep in with different cores and expected performance, read:
- G3TXQ web site contains a very good article (http://www.karinya.net/g3txq/unun/) on cores and ratio comparison for 9/1 and 4/1 un-un.
- G8JNJ web site has a very good article (http://g8jnj.webs.com/cometcha250b.htm) on building a clone of the Diamond BB6W wideband antenna (scroll down...). The balun used is a 6/1 but there are some interesting facts and tests on various cores.
Building the 9/1 Un-Un
My requirements was:
- 80m to 6m use
- QRP or max 100W power
- light and little for portable use
The SWR of Un-Un with a 470 ohm dummy load was measured and better than 1.3:1 from 3.5 to 50 MHz..
I've made several 9/1 for me and for some friends using different materials. On-air performance is sufficient, not bad for a so simple antenna.
Cores used in the Un-Un :
- Amidon toroid (T200-2, T130-6, FT140-43, FT240-43)
- Unknown Yellow-and-White toroids (I suppose are for switching power supply)
- Ferrite sticks from old AM radios
In order to chioose the core material, mind this simple facts:
- Using Amidon cores (in particular TXXX-2 and -6) the losses are relatively low and the performance of the antenna is better, but in the lower bands you may need a antenna tuner to obtain good SWR. Otherwise, if you don't like to bring ATU, try to use a "long" coax feeder (more than 10 meters)
- Using low frequency toroids or ferrite sticks the losses incrase a lot, but this lower your SWR and you don't need an ATU at all.
Look my new "Shorted" longwire antenna