Don't get me wrong, you can attach a high-quality mic to your smartphone if you want to, but just be aware your guest will seem washed out standing next to your high-quality voice.
A good digital recorder doesn't cost as much any more. Now-a-days it's mostly used among professional musicians who are playing around with a tune and don't want to rent out studio space to test it out. This is great news for you who may not want to spend their entire savings on portable audio equipment.
Good recorders on the market today are the Tascam DR-05 and the Zoom H4n. The Tascam sells for about $100 on Amazon and the Zoom is going for about $200. The Zoom is the pricier among the two because it has four channels that you can record on simultaneously. They both have pretty great microphones built-in, but the reason why we don't want to use those for our podcast is because the audio levels will vary if two people are sitting far enough away from each other.
You will have more control over the audio quality with two good lavalier microphones, which brings us to our next topic..
These are tiny clip-on microphones that you attach to your shirt. They will make you feel cool, professional, and snazzy. There are microphones out on the market that come with a battery pack and are wireless, but I think the moment you have to start dragging around extra equipment, you reduce the meaning of what 'portable' is. So for today, let's keep it simple.
There are two great microphones on Amazon that come highly recommended. The first one is the Audio-Technica ATR-3350 valued at $33 dollars and the Sony ECMCS3 Clip valued at $20. The Audio Technica is the preferred choice because it has a 20ft cable. This gives you the flexibility you need for interviewing people at a distance. The Sony Clip has a 3ft long cable, so it may make things a bit awkward for interviewing purposes. However, if you are fixed on the Sony Clip, you can buy a jack extension cable for about 8 dollars or less.
The Two-Way 3.5mm Jack.
This jack is a necessary component to tie the entire recording process together. As you can imagine, you place your microphone in one end, your interviewee's microphone in the other, and then you plug the male end into your digital recorder...and you're done! This is what it takes to obtain a high-quality recorded podcast on the go. The microphones listed above are great for both outdoor and indoor activities, so you'll never feel left out of the loop again.
You can purchase jacks that have more than two inputs. For example, if you're expecting to interview a group of people all at once, you'll need the appropriate amount of mics and to attach it into your 3 input, 4 input, or even 5 input 3.5mm jack. If you are expecting to interview even more people in one room, then using the built-in microphone that the Zoom H4n has, although not ideal, will help you get there.
Overall, owning two mics, a tiny portable recorder, and a jack split is the best option you can have to get great results on the go without being bulked down by equipment. You'll be able to produce a great sounding podcast with your interviewee and be proud to share it to your listeners in no time. Editing your podcast on the go might be a little bit difficult, but don't worry, you have Podfly here to help you.