Hello,
I am new here, and I have a question. I have a Spitfire, Part 15, AM Transmitter I bought off eBay. I have tested it and it works fine.
I have run into a problem. I live in a subdivision that is under a Homeowners Association, and I am afraid to put up anything outside. My house is a two story home with the attic at the top, which is the highest point of the 2 story house is.
My question, is there a coil loaded antenna I can use and hide it? A engineer told me a coil load CB Antenna would work, even though it was designed for the 26-27 Mhz range. I don't want to waste my money on an antenna that won't work that is not turned to the 1.6 Mhz Medium range.
If anybody has any ideas, pass them along.
Thanks in advance
if you have about 11ft clearance in the attic then a base loaded (linear air wound coil) 3 meter monopole over a large symmetrical ground plane laid out over the attic floor radiating outward from central point at base of monopole would be your best bet for good range. place Tx at base of monopole for lowest loss. look at the manteca magnum or on the sstran page for low loss antenna designs. they can be modified by adding inductance to reach your 1400 khz frequency you have previously mentioned over on the other forum. although for efficiency i would consider using 1700 if it is a clear frequency as 1400 would present a lot of loss vs 1700. if you don't have 11ft of clearance then use the same design as above and make vertical to the peak of roof then bend radiator to an "L" at peak of roof and run remainder horizontal.
Horatio: An engineer told you that? It's a long ways between the high end of the MW band and the high end of HF. There is not enough inductance to resonate a 3 meter whip at the top end of the AM band.
If this transmitter is as described on the website below, then it includes an "ATU" to match to a 3-meter conductor used as an antenna.
http://www.vcomp.co.uk/spitfire/spitfire.htm
it does include a cheap high loss matching coil but it can be bypassed in favor of a 50 ohm lower loss external base loaded system.
How good it is that Horatio Caine is getting the very best suggestions for indoor antennas. Here's more...
I use an "Wintenna," a name I coined to describe an indoor/outdoor antenna based on a metal window frame. If you have even one metal window frame or metal screen frame you might try it.
Wintenna -- My AMT3000 from sstran.com is located on the floor underneath a corner of a desk that has an outdoor window right at that location. A self-made loading coil is located directly next to the transmitter oriented horizontally so it aligns with the baseboard. It was made using the instructions kc8gpd mentioned at the sstran website. From the coil a 1.5-foot wire is stuck to the wall using a glue stick, running vertically up to the window, underneath the closed window, and attached with a self-threading screw to a hole drilled in the lower center of the outer metal window frame. The window frame becomes part of the antenna, about 6.5-feet vertical, and outdoors at the top of the metal frame another screw attaches another vertical wire that goes up until it all adds up to 3-meters.
The transmitter ground drops down behind the baseboard and is clamped to the house's I-beam which is both electrical ground and cold water pipe ground.
The AMT3000 has special modifications required to achieve the needed impedance to make it all resonate, and I get a solid indoor/outdoor signal.
By the way, I am working toward an AM indoor antenna project, which will be posted at my website:
http://www.kdxradio.com/intenna.html
Rich has a good point. Does the Spitfire allow you to use an external base loaded antenna (for example, on an AMT3000 or 5000, you have to bypass or leave out the components that tune a 3 meter non base loaded antenna, and on the Talking House there is a switch). The Spitfire does seem to have an internal antenna tuner, so you'd have to bypass that to use the coil antenna.
If you can't, then you might as well use the included wire antenna, and string it as vertical as you can inside the attic - if you have the clearance, you can replace the wire with either a whip or a pipe to get slightly better performance.
Yes, the Spitfire has a built in bypass for an external antenna.
Using the Spitfire Link provided in Post # 4 by Rich(F), I could find no specifications at all describing any details about the Spitfire.
Where does it mention an ATU?
What is the RF output impedance?
Making suggestions about antennas is rather useless without that key information.
Does the Spitfire allow you to use an external base loaded antenna (for example, on an AMT3000 or 5000, you have to bypass or leave out the components that tune a 3 meter non base loaded antenna, and on the Talking House there is a switch).
Just to note that no matter if the loading coil needed to resonate a ~3-meter vertical monopole is located within the transmitter enclosure, or external to that enclosure in the form of an air-wound coil at the base of that ~3-meter monopole -- both of those monopoles in fact are base-loaded.
The Spitfire does seem to have an internal antenna tuner, so you'd have to bypass that to use the coil antenna.
Probably it would be possible/practical to use the built-in "ATU" in the Spitfire to match it to an external 3-m "antenna" conductor without needing to bypass that ATU in favor of an externally-located loading coil installed at the base of that ~3-m radiator.
Most likely the same is true for the AMT5000 when used in this scenario.
Assuming the Spitfire has some sort of internal loading coil (ATU) as a toroid wound with small guage wire (24 or smaller) how much difference would there be using an external coil made with a larger guage wire (14 or larger)?
Larger wire, less resistance, less loss?
Larger wire, less resistance, less loss?
That also depends on other characteristics of that coil (winding pitch, length, diameter, frequency etc).
But regardless if ~ 3-meter monopoles are resonated in the AM broadcast band by a loading coil using a "small" OD conductor wound around a ferrite core located within the transmitter enclosure, or by a "large" OD conductor, air-core coil -- BOTH such monopoles are base-loaded.
Do you feel it would be worth the trouble to bypass the internal coil and use an external coil or would the gain realized be insignificant?
I realize it would be difficult to give a solid assessment not knowing how the Spitfire internal coil is made but in general, what do you think one could expect in terms of the radiated field when comparing a poorly designed coil to one that is optimized?
That is a serious spec sheet, Rich(F). Thanks for posting it.
I tried finding the spec sheet two times before declaring defeat, maybe it's a difference in browsers... I use Firefox.
It sure looks like someone could do serious part 15 business with the Spitfire.
But indoor antenna wise, where do we stand?
BACK FOR AN ADDENDUM
Wondering what, why and where, I returned to the Spitfire site. This time I scrolled down and viola! The spec sheet!
Rather than being embarrassed, I wish to thank everyone who overlooks my apparent scrolling handicap.
The Spitfire used to be the old Gizmo transmitter! So it says.
Carl Blare wrote: Using the Spitfire Link provided in Post # 4 by Rich(F), I could find no specifications at all describing any details about the Spitfire. Where does it mention an ATU?
Rather complete specs were given in that link when I Googled and quoted it -- including references to its included, internal ATU. Below is a clip of its specs as stated on line, and acknowledged by other readers of this thread.
For those who didn't/couldn't access it, the specs including the text about the ATU are pasted below:
_________________________
Spitfire Kit PLL Medium Wave and 160M Amateur Band AM Transmitter US 10KHz Channel Spacing
The Spitfire transmitter is now available as a kit...at an affordable price...with the performance and features of units costing many times the price...!
The Spitfire employs a crystal controlled frequency synthesizer for accurate tuning and drift free frequency stability. The synthesizer tunes the entire AM broadcast band (500 - 1800 kHz)
The Spitfire provides the full 100 mill-watts, as allowed for FCC Part 15 operation without a license.
The Spitfire employs an updated RF output and modulator stages with new features that results in very low distortion modulation right up 100%, and an almost indestructible output stage.
The Spitfire has a two channel mixer for either Stereo to Mono or twin channel Mono audio sources.
The Spitfire has a totally new ATU (Antenna Tuning Unit) which has an variable capacitor to peak the network to match a 3 Metre Antenna as specified under Part 15 rules.
The Spitfire printed circuit board uses a grounding technique that splits the RF and Audio grounds, as well as providing and external grounding point. In addition a new external "universal" power supply provides regulated DC power to the transmitter.
Fully Built and Tested: High Quality PCB, Plastic CASE to IP56
Specifications
EU R&TTE directive and EMC Directive 2004/108/EC - in order to purchase this item, you must agree that:-
1. You intend to export the item to an address outside the European Union. Under no circumstances will this item be resold or put into service within the European Union.
or
2. The item will only be used within a completely RF screened and shielded metal enclosure or environment for testing, evaluation, experimental or laboratory purposes and any other use that does not radiate electro-magnetic waves into free space. I am not an unqualified end user and I have RF engineering and electromagnetic compatibility skills at my disposal.
or
3. For use by a Licensed Radio Amateur on 160M Band