I've been looking over the info in the library about coils to match the transmitter better to the short antenna. Most info there is about air core coils wound on PVC pipe.
My question is, would it be OK to use a ferrite core, (common misuse of term - iron powder NOT ferrite) either a rod or a torroid, to wind the matching coil? It would require a lot less wire winding to get the same inductance, and would seem to have less resistance because of that - has anyone tried this?
I've been looking over the info in the library about coils to match the transmitter better to the short antenna. Most info there is about air core coils wound on PVC pipe.
My question is, would it be OK to use a ferrite core, (common misuse of term - iron powder NOT ferrite) either a rod or a torroid, to wind the matching coil? It would require a lot less wire winding to get the same inductance, and would seem to have less resistance because of that - has anyone tried this?
Also, does any one know of a reason a ferrite core shoud not be used?
TIA
Parts availability is probably why most Part-15 antennas are built with air-wound coils. You can get all the materials at your local hardware store or home center.
Frank
Part15er: I'm going to ask a friend to provide me with a "simple guide to ferrite coils" with a focus on the little under 10' antenna we get to use. I'll post that here and see if Kevin will be kind enough to drop it into the library.
Ok - here's the info regarding the Toroid Inductor:
[[i:3b8ae508d3]This has been slightly edited from an email with an engineer I know who has helped me build MLR[/i:3b8ae508d3]]
Sure that can be done, but it won't yield as high efficiency as the air coil. With a toroid coil, smaller wire has to be used; the wires resistance lowers the Q of the coil. Then there are losses in the ferrite as well. The best Q for RF frequencies is with air coils and large wire.
That said: I use a FT-240-77 core for the low power AM transmitter that resonates with the 30pf antenna (a 108" whip). That works out to about 130 turns of #24 gauge wire on the FT-240-77 toroid at 1700. A 108" whip is 30pf and 9' of the 1/2 inch copper pipe is 34pf. To calculate the inductance the formula is 1 divided by 39.4 x frequency (squared) x the above capacitance. The toroid comes with a chart of wire size vs. turns for a given inductance.
NOTES: The type 77 core saturates at about 7 watts.
Get your core here:
http://www.amidoncorp.com/aai_ferritecores.htm
More information on cores:
http://www.amidon-inductive.com/aai_ironpowdercores.htm
http://www.njqrp.org/mbrproj/zmatch_original.html
http://www.eham.net/forums/HomeBrew/1371
Thanks frank19 and mlr - looks like it's a bit of "6 of one, half dozen of the other" and the air coil seems like it would be cheaper - I checked those amidon prices and it can get into some bucks if you buy a larger ferrite rod or torriod.
Looks like the air coil has a bit of an advantage, unless you're cramped for space and want to put the coil insdie the transmitter enclosure.
Thanks again! 8)
I have a transmitter with the coil inside and it uses the rat shack steel whip - and i also have the one where the tx is attached to the air coil. I think the air coil gives me more range and better sound .. Now.. I will say, this could be my imagination 🙂
😀 I was reading the below post about the 108" whip being 30pf, while the copper antenna is 34pf... my question is when you tune the copper antenna by raising or lowering it are you changing the capacitance of the antenna from 34pf to say 30pf when you're done tuning it? I'm using the 108" whip with the air coil inductance to match it, it's working properly or does the 108" whip need to be cut shorter to match the air coil? I know that the Rangemaster has a tuned toroid coil to match the 108" whip... other than using an SWR to tune the antenna, what is the better method for such low wattage that we are using to tune the antenna besides the light bulb method?
ABOUT ANTENNA CAPACITANCE:
Antenna capacitance is a function of both the length and the diameter of the antenna. The RadioShack CB whip is actually 102" long and .2" in diameter. This calculates to be 24.5 pf. A 102" long 1/2" copper pipe (actually .625" OD) calculates to be 30.3 pf. These are calculations. Actual measured capacitance may be slightly different.
If the copper pipe antenna is length adjustable by plus/minus 4 inches you will get 29.3 pf at 98" and 31.3 pf at 106". When used with a loading coil with taps spaced properly, the whole antenna can be tuned by selecting the best tap and then peaking the tuning by adjusting the antenna height.
If your transmitter doesn't have an internal test point for a voltmeter to show the tuning peak, the absolute best way to tune is with a Field Strength Meter (FSM). An FSM has a short whip antenna. It should be positioned as far as possible from the antenna within range to get a usable reading and no closer than about 2 ft and should be supported in a stable position. The meter will read highest when the antenna is tuned to resonance.
MFJ sells these inexpensive FSMs:
http://www.mfjenterprises.com/products.php?prodid=MFJ-801
http://www.mfjenterprises.com/products.php?prodid=MFJ-802
http://www.mfjenterprises.com/products.php?prodid=MFJ-802X
Phil B
ABOUT ANTENNA CAPACITANCE:
Antenna capacitance is a function of both the length and the diameter of the antenna. The RadioShack CB whip is actually 102" long and .2" in diameter. This calculates to be 24.5 pf. A 102" long 1/2" copper pipe (actually .625" OD) calculates to be 30.3 pf. These are calculations. Actual measured capacitance may be slightly different.
If the copper pipe antenna is length adjustable by plus/minus 4 inches you will get 29.3 pf at 98" and 31.3 pf at 106". When used with a loading coil with taps spaced properly, the whole antenna can be tuned by selecting the best tap and then peaking the tuning by adjusting the antenna height.
If your transmitter doesn't have an internal test point for a voltmeter to show the tuning peak, the absolute best way to tune is with a Field Strength Meter (FSM). An FSM has a short whip antenna. It should be positioned as far as possible from the antenna within range to get a usable reading and no closer than about 2 ft and should be supported in a stable position. The meter will read highest when the antenna is tuned to resonance.
MFJ sells these inexpensive FSMs:
http://www.mfjenterprises.com/products.php?prodid=MFJ-801
http://www.mfjenterprises.com/products.php?prodid=MFJ-802
http://www.mfjenterprises.com/products.php?prodid=MFJ-802X
Phil B